RS/6000, AIX problems

[Popi Havaki]

Hello,

I am a new user of a RS600/320 machine running AIX OS. I have some problems
running fortran programms in the background successively.After about the 
tenth running i get the message :
1501-222 Cannot fork processes,Resources temporarily unavailable ..

Does anyone know anything about that ?? Is this a problem of AIX or
i have limits using system resources ?? How can I solve this problem??
Any ideas??
                             Thanks, Popi havaki