Why SUID Binary exploit does not yield root shell?

[Kryptik Logik <kryptiklogik () hushmail com>]

Folks:

I've managed to find a buffer overflow and exploit it to exeve a /bin/sh 
using my payload shellcode. However, whenever I run my exploit, I do get a 
shell but just that it is an ordinary shell under my account (as id would 
indicate).

The binary that I've exploited is suid bit set so theoretically shouldn't 
it create a root shell? I've tested my exploit on a small sample 
vulnerable program that I wrote with the exact same permissions as the 
binary in the system and I could get a root shell!

What is the magic here (if any)?

Thankx in advance,

# klogik