Yes, taller enclosures are tougher to maintain humidity.

What type of hygrometer are you using? If it's a dial style analog one they are wildly inaccurate (up to 15% off at times) and basically useless.

Having a decent size water bowl helps too.

The biggest thing I can advise is to wrap your enclosure in 1/2" foam board insulation. I use this on all my glass enclosures and it helps immensely with heat and humidity control.