Weird shellcode behavior

[gljuposti () gmail com]

I am building an exploit (heap OF) and I am experiencing different shellcode behavior depending on the parametar I can 
in no way associate with this problem. For one parametar value the shellcode gets  executed correctly, but for the 
other, there are problems. 
For example, the calc.exe shellcode creates a calc.exe process (I can see it in task manager), but its window is never 
displayed. Some other shellcodes like the add user shellcode don't work at all. I use shellcodes from Metasploit.
The shellcode gets called each time and it hasn't been changed in memory, it just doesn't behave like it should.
Has anyone ever experienced something similar or has any idea why it could happen? This leaves me totaly confused.
TIA