The potion of master thievery (or potion of perception, IIRC) can help - but first, are you using the thieving skill tool - the same one you use to pick locks? You have to use that one on a detected (red highlighted) trap, and you will - depending on the skill check- get:
Trap disarmed
Trap not disarmed
So, are you getting Trap not disarmed messages? If so, your skill is inadequate. If not, you're likely using the wrong tool.
Hope this helps
