I'm going to assume you know how to run a gel and are able to prepare a sample without degradation of your proteins. In that case, fuzzy bands can be due to post-translational heterogeneity in the protein. The most common  post-translational modification  that will cause bands to be fuzzy is glycosylation. Glycoproteins often have variability in the attached polysaccharide, especially sialic acid, which will cause charge variation. Lipidation, phosphorylation, etc. also can cause heterogeneity. All of these can cause bands to appear fuzzy. The big question concerning your own gels is whether all the protein bands are fuzzy, or just some. If just some are fuzzy, you should consider whether they are glycoproteins, etc. If all are fuzzy, you have some systematic error, as has been addressed by others. The interesting thing about glycoproteins and other proteins with charge heterogeneity is that a discontinuous buffer PAGE system (having a stacking gel) actually causes the fuzziness, i.e. the stacking gel separates the variant species of the protein into close, separate bands, making it look like one diffuse, or "fuzzy" band. Using a continuous buffer PAGE system (no stacking gel) will yield a single band, but in such a system you have to keep your sample volume to a minimum to get a sharp band. One good continuous buffer PAGE system is the Fairbanks system (as opposed to the widely used Laemmli system that is discontinuous, using a stacking gel). Hope this helps.

More likely due to sample volume too high or overloaded.

As a solution, load sample in less volume, as a tight layer in the bottom of well. Height of the stacking gel beneath the well should be at least two times the height of the sample in the well. Samples may be degrade keep them on ice after boiling till loading. • Concentrate the samples if they are diluted before loading them.

Please check the pH of Buffers and if possible filter them before use. Also use fresh running sds buffer

Can you provide more of a description of your problem (and maybe even a picture). Are these fuzzy bands during image capture? Or at what point are you noticing the problem?

Please refer attached doc Bhagya. it might help u to trouble shoot with SDS-page.

In SDS-PAGE pH is the most important point. which depend for gel polymerase  movement of the protein. sharpness of bands. Next is solution preparation. Freshly prepared solution works better. Some solution you can keep it at 4 degree centigrade for 3-4 time uses. Running buffer should be fresh. These points will take care of your problems. 

There could be many reasons, however a precise question could help us to troubleshoot your problem. Still Check 

1. PH of both stacking (pH 6.8) and resolving gels (pH 8.8).

2. Loading Volume (Use less volume or more concentrated protein suspension)

3. Check the temp of running buffer (High temp. could result in smile effect)- For this check this conversation on RG https://www.researchgate.net/post/How_can_we_reduce_or_nullify_the_Smiling_Effect_in_Polyacrylamide_gels

Dear Bhagya,

There are already some excellent suggestions. I would like to add few more things. I think the major reason for not getting sharp bands is the improper electrophoretic mobility. Which could be due to many other reasons. So, prepare the stacking and resolving gels more carefully maintaining the pH. The proteins won't be properly stacked if the pH isn't well maintained. Allow it sufficient time to stack as suggested by Wai Chin Chong (about the length of the stacking gel below the wells). As already mentioned, your proteins won't separate nicely if resolving gel's pH is altered. Apart from accurate gel preparation, you can pre-run the empty gel for 10 mins before you load the samples. That prepares your gel for the actual run (kind of a warm-up which ensures better electrophoresis)! You can then use a high voltage to run your samples. The lesser voltage, the slower it will run and the proteins will have a chance to diffuse outwards, so its important to maintain the constant driving force that will drive them in one direction only (positive electrode, downward) and will minimize the chances of movement in any other direction (outward) and produce fuzzy bands.  About sample preparation and loading try to use lesser volume and high concentration as suggested. 

Hope it helps!

I'm going to assume you know how to run a gel and are able to prepare a sample without degradation of your proteins. In that case, fuzzy bands can be due to post-translational heterogeneity in the protein. The most common  post-translational modification  that will cause bands to be fuzzy is glycosylation. Glycoproteins often have variability in the attached polysaccharide, especially sialic acid, which will cause charge variation. Lipidation, phosphorylation, etc. also can cause heterogeneity. All of these can cause bands to appear fuzzy. The big question concerning your own gels is whether all the protein bands are fuzzy, or just some. If just some are fuzzy, you should consider whether they are glycoproteins, etc. If all are fuzzy, you have some systematic error, as has been addressed by others. The interesting thing about glycoproteins and other proteins with charge heterogeneity is that a discontinuous buffer PAGE system (having a stacking gel) actually causes the fuzziness, i.e. the stacking gel separates the variant species of the protein into close, separate bands, making it look like one diffuse, or "fuzzy" band. Using a continuous buffer PAGE system (no stacking gel) will yield a single band, but in such a system you have to keep your sample volume to a minimum to get a sharp band. One good continuous buffer PAGE system is the Fairbanks system (as opposed to the widely used Laemmli system that is discontinuous, using a stacking gel). Hope this helps.