If a root canal is performed & the filling that was placed to fill the drill spot isn't fully filled, can that cause problems?

Yes: If there is leakage around the filling in a tooth with a root canal, you can get bacterial contamination in the root which may cause reinfection and may require retreatment of the root canal.

Possibly: You may feel a tiny depression with your tongue.. That does not mean it is not filled correctly. Is this a temporary filling or the final filling? Any questions regarding the integrity of the restoration should be addressed to the dentist who placed it.

Yes: It can. You don't want bacteria to get down into the filling material placed into the canals. See a dentist to make sure it is filled properly.