Do you know if one cat is using both litter trays and putting off the other cat? May be worth keeping an eye on this, if it could be the case.
Also if you do not do so, try cleaning it each time it is used. May be a hassle but if she feels it is dirty, it could be putting her off.
One of my cats started to refuse to use the litter tray and I managed to solve the problem by constantly putting her in it after she had eaten. She then got the message and started to use it again!