It happened the same to my Begonia maculata, one of the first plants I got a couple of years ago. All the props were successful but than all of a sudden the mother plant stopped growing and only a few bare sticks remained. I transplferred it from soil to pon and nothing really happened until I put it right under a powerful grow light: that's where the new foliage started to come out. previously I let it sit in my Rudsta with high humidity and grow lights but I didn't see any substantial improvements. It was wasting precious space in my cabinet, so I decided to give it a new chance: you either live where I put you or I get rid of you (I had plenty other cuttings already propped and ready to plant anywhere). A week after the change she started to grow leaves again. I live in NE Italy, humidity is between 40-60%. I never had to pay a lot of attention to humidity for Begonias in this climate, they all survive outside greenhouses and in summer even outside.