Human antibodies dilution problem

Dear Flowers,

I wanted to know how people stain for several antibodies at the same  
time when the antibodies have been previously diluted. For example,  
human antibodies from beckman coulter come  20ul/test, so if I stain  
for several antibodies at the same time (in the same well) my  
experience is that the antibodies get diluted too much and the % of  
stained cells is not the same as if I would have stained with a  
single antibody per well. Has anybody else noticed this problem and  
what can I do to solve this issue.

Thank you very much for your help,
Jenny Zilberberg