Welcome to the contrary world of hamsters Sarah
Are you using the potty litter balls or chinchilla sand? try putting a bit of the litter in each corner and you will find she will use one more than the others and then slowly you can only put the litter in the one corner - moving it into a potty holder too eventually. Alternatively put a potty jar in each corner! Some hamsters love to eat, sleep, and, pee and poop in their potties, gross but they enjoy it
You don't need a fancy potty in each corner a jam jar will do or an empty food dish etc something heavy she can't move out the way is best.