Welcome aboard Subsim.
Sounds like you need to clean your sensors, wires and buttons. Clean out the inside of the TDC while you're at it. Get on that right away.
Apparently you're using auto targeting. You say you're making sure the torpedo doors are open. Are you "locking on" with the auto target? If you don't, then your "crew" won't make any changes and the gyro angle will remain the same. Lock on and your "crew" and the TDC will continue to update the information and you will probably get an accurate hit.