Two possibilities are : (1) The bacterial strain you are using is not susceptible to the antibacterial compound, (2) The compound is so insoluble (or unstable) that it was unable to diffuse out of the well.

IS your sample showed antibacterial susceptibility earlier?? 

if no than may be it has no antimicrobial activity.

if yes than there might be some changes in  conditions media etc, u must look for it

may be low concentration of samples , try to do MIC and MBC 

with my best wishes

1 - Are you using an antibiotic that is supposed to be effective against the species of bacteria you are growing?

2 - What antibiotic are you using?  Is it in the correct form to have antibiotic activity (ie. not needing to get into the stomach and intestine and be absorbed)?

3 - Are you getting a sufficient level of antibiotic diffusing into the agar to affect he bacteria (ie. is well filled, or is level below the surface?)

First, please control the  bacterial load that you distribute on plates.

Second, some batches of antibiotics are not stable on the discs, others should be kept frizen or refrigerated.Discs can be easily inactive.

Try to test different batches recently open and confront results with antibiotics of the same family.

Actullay im testing a plant gum, that is said to have antibacterial potential. I have tried dilute as well as concentrated samples but it showed no responce .. And im also trying Ag  nanopartiles but they also not showing any activity .. I wonder silver nanoparticles antibacterial potential is reported by same method I have used to prepare them but when i use those nanoparticles against my strain they dont show any antibacterial activity.. then i used silver nitrate simple solution it showed strong atibacterial activity ! help me through this

Disk diffusion assay may not be the best test, particularly for nanoparticles which are likely to get caught in the paper support. Maybe try using a known antimicrobial as your negative control (kanamycin, neomycin, tobramycin, tetracycline, etc. Something that you know will affect that strain).

i think you first  make a servay. ask different question from the local people about your plants are used by tradional way. if from sarvy you confirm this plant use a medicnal. than you try to incressed the quntity of solution.

Are the plant gum and silver diffusing through the agar?  If they are in a form that cannot diffuse it would explain the results.  Try another means to dissolve them and see if that allows for some antibacterial activity.

thanks Robert Sir ! I think the problem is with diffusion .. sample is not diffusing !

if diffussion or inhibitors is problem use MULLER hilton agar for that.  as para-aminobenzoic acid (PABA) and thymine/thymidine content in Mueller Hinton Agar are reduced to a minimum, thus markedly reducing the inactivation of sulfonamides and trimethoprim

It will be better if you can first standardize your bio assay process with standard antibiotics. The antibiotic and blank discs are also available to avoid preparation of wells.

Vijay Singh do you have a relevant literature related to the method or changes you are referring to ! if yes then please mention those ! 

The no zone appearance in Diffusion agar testing it would be:

1. The sample to be tested: Diffusion method is water mediated, so it should be better solve in water;

2. The killing capacity of the tested sample it would be below the minimal concentration to the killing to bacterial control/tested.

3. SHould be in mind: what antibacterial content in the sample so you will test the sample??  as herbal material that done by some study,  that some time was not clear what the content??, should be clear

best for you

yasir .....here is a relevent link for you acc to CLSI guidelines... http://www.sciencedirect.com/science/article/pii/S2095177915300150

CLSI, Methods for Dilution Antimicrobial Susceptibility Tests for Bacteria that Grow Aerobically, Approved Standard, 9th ed., CLSI document M07-A9. Clinical and Laboratory Standards Institute, 950 West Valley Road, Suite 2500, Wayne, Pennsylvania 19087, USA, 2012

Try for LB study this is a very simple method for testing antimicrobial activities whether your materials are working or not.