Using multiple processors

[n00b]

Hello.
My program has a great deal of computation to do, so I thought I'd 
create several threads in order to use multiple processors. It worked 
fine with a simple "test" program, but when I try to do this with the 
real program, only 1 processor is used.
I'm using D1 and windows 7 64bit.
What could cause that difference? I don't use synchronized methods, 
diffrent threads don't access the same data ( every thread gets a slice 
of an object array with a "compute()" method to be called ). I tried to 
disable the garbage collector and reduce memory allocation/deallocation.

Thanks for any help.