beta gal assay

[Gopal]

Hi I have a question on beta gal assay. I was trying to test for
interaction between two genes using yeast two hybrid system. In the beta
gal assay I was getting green signal rather than blue. My contol strain
and the colonies having known interacting proteins also give the same
color(SNF1 and SNF4). I used two strains (Y153 and Pj69-4A) and changed
buffers. I made sure the buffer pH is 7. Can someone please tell me what
could be the problem.?
Thanks
Kovvali