They are very active fish - so much so, I had started worrying my 75 wasn't big enough, cruising back and forth all day in front of the glass!
She should get to like frozen foods, they have a ravenous appetite. However, the main food should still be algae. Mine never took a liking to algae strips - they weren't "stringy" enough, I guess.
I had tremendous luck growing tons of caulerpa algae in my fuge, and he went wild for it. I'd throw in a handful of it every few days and he'd spend some time hunting it all down in the tank.
As for the ones in the store, they seem to be very shy when in new/uncomfortable surroundings. Mine would always come out for me, but if I had a guest, he'd hide!!
As long as her belly looks fat and there's plenty of greens avail, she should be fine.