EXS 24 Logic Sampler Users Group

I have a mix going of 3 audio instrument s and 3 EXS instruments , but for some reason  I 
can't hear one of the esx instruments , i can see the level moving but no sound , why is 
this , it happens quiet a lot , somtimes more than one instrument is lost , it's very 
annoying , can anyone tell me if I'm doing somthing wrong , i've checked al the levels , 
help !!

thanks in advance 
r