KMG has a lot of good tips. It has to be something subtle, you're definitely not doing anything obviously wrong. If the surface temp of the hot side or hotspot is low 90's you could try bumping up the temp just like 2 degrees. Balls tend to be quirky and sometimes little changes work. At least she eats and rats at that! Worst case scenario it is just her preference and she grows a little slower. Try everything one at a time and good luck!