You may be dialing the combo with one of the numbers slightly off. Stand directly in front of the dial. Verify the numbers that you are using. In most combo dial locks, there are two marks on the dial ring. Use the vertical line at the 12 O’clock position.
If you experience continued difficulty, this may be a sign of a problem developing. Contact the factory for a recommended locksmith to service the lock. This will prevent a lock failure and drilling the safe.