Could be many reasons. Did you narrow down the potential problems? I guess this issue is not related to the bad reagents, or improper method. If you have excluded every possible reason, it could be the antibody quality. Sometimes, the specific batch of antibody doesn't work.  Also, you can contact with someone who published a paper in which MDM2 antibody was used. Good luck.

Hard to say what's going on from the limited information you have posted. I always liked to use rabbit polyclonal Abs rather than mouse monoclonal Abs. I would try rabbit polyclonal first, like this one:

http://www.abcam.com/mdm2-antibody-ab38618.html

Thank you for your replies Fei and Anton. To be more specific, I meant to ask if one need to use MG132 (Proteasome inhibitor) or Nutlin3 to facilitate the detection of MDM2 in cancer cell lines. I agree with Fei that the antibody could be from a faulty batch, because this particular antibody has been widely cited and considered one of the best options available. I would try the experiment with using MG132 before ordering a new one.

I have no experience with MDM2 western blot, so all my sayings could be wrong. Based on your information, MDM2 is not stable with a quick turnover mediated by proteasome. Is it common to use MG132 in the detection of MDM2? I am worried that an extra treatment is introduced into your experiment, it may affect the interpretation of your result. But it is definitely a good try with MG132 treatment at the first.  Good luck. 

@Fei: The papers I have read so far seem to be using MG132. In some figures, I could clearly see the marked increase in the levels of MDM2 following MG132. I agree with your point that an additional treatment may give an extra layer of complexity. Will try MG132 treatment before trying a brand new antibody. Thanks.

 Hi Harsh,

There could be a lot of reasons, may be you are loading less quantity of protein (load 50-80 ug of protein), Change the dilutions of the antibody or if possible use antibody from different company. 

Good Luck

Yes, there are manny reasons to avoid detecting of proteins. May be you get some cells from a friend producing MDM2 for sure as a further control. Good Luck.

For anyone who is still looking at this question, I just used higher concentration for the primary MDM2 antibody (1:100 or 1:200), and I could detect it by WB readily.