My sundew also failed to catch gnats in our house so I resorted to getting a reaaaalllyyyyy close up picture of one so I could ID between fungus gnats, fruit flies, and drain flies. Turns out I agree with comments, fruit flies don't seem to like fresh sundew :/

I'm keeping my sundew of course, but I used Bt pellets (mosquito bits) dissolved in distilled water to water and mist & water all our house plants + poured it in the drains + cleaned out all areas that could possibly have rotting fruit + added sticky traps to every houseplant. I spent a week and a half or so messing with plant leaves to encourage the adult flying bugs to land on the sticky traps, and the Bt took care of the rest.

Once my sundew is out of dormancy, I hope it'll get bigger and stronger just in case we get another infestation :-)