Are you sure that it's not being dispelled by the enemy. Best way to test it is to cast it without any enemies in sight. The spell should last for one turn which is one minute of real time. Within this turn there will be 10 rounds, so you should see the true seeing message 10 times.
Only other thing I can think of is that your spell is corrupt so I've extracted mine which I know works fine. Save this
target to your BG2/override folder