The same strong bands with different primary antibody - (Apr/09/2012 )

Hello all,
Does anybody have any idea why I can have very sharp and strong bands on the same size (around 20kDa) every time with the same samples with different primary antibodies? It's happen with the same set of samples but, for example, only in the sample No 2,3 and 5. But not in the rest of them. I tried different primary ab and it gives me signal with every secondary ab. Is it protein degradation? Or what it can be? Very appreciate any suggestions! this forum is great!

What are your samples (e.g. lysate, IP, purified protein...)?