Before you even think too hard about drilling, find out the size of the hole !!!!!
If the hole is in a specific size range ( and not too small) your problem may be blockage (constriction in the gas tube) or the gas block might be covering some of the gas port hole. There might be an issue with the gas tube to bolt carrier mating surface causing a leak that you cannot see with the naked eye.
I would suggest doing a canned air test before you do a teardown of the rifle. Also mark the position of the gas block before it is taken off, then look for carbon build up on it and on the barrel by the gas port hole. This may show a misalignments of parts.
Añyway just throwing that out there , because once its drilled bigger there's no going back ... Only band aids to fix your new problem.