Neither my Desert Tortoises nor Russian Tortoises like cactus. I thought this was a favorite, especially of the Desert Tortoises. I keep offering it to them every few months. They take a bite or two and do not seem to like it.
How old are they? My leopard never liked it until he was around 17-18 months. He still only eats a couple bites of it. I will keep trying though. The more variety the better.
I'm working on my Russian ( age unknown) and my Sully, ( a stunted four year old i picked up last year) I offer, they refuse. I figure one of these days they'll realize it's delicious healthy food
Tortoise are creatures of habit, and also can be like spoiled children. If they aren't familiar with a new food they often won't eat it until they see it every day for a month or more. You could try finely chopping it and mixing it in with their other more preferred foods. If they are just being picky and they prefer their store bought lettuce, then a little hunger might cure their pickiness.