Have medical causes have been ruled out? How long has this been going on (can you think of any event around the time this behavior manifested?)
Info request = info gathering. Any expert can reply.
You may not believe that it is related (and a percentage of it it may not be) but kids like routines and what is predictable. Her behavior may be in response to your mom not being there and she's feeling hopeless or anxious (or both)
Whatever your mom did while she was there during the potty time will be something you may want to implement/recreate. Whether it it s lot of praise, rewards when she finely goes (but never punishment when she does not) and then talking to her about how girls growing up do this on their own is going to be important.
The workers at her day care should not show frustration when she does not do it. She should not feel guilty or shamed. The dippers idea is something you can revisit for the time being. When your mom calls, have her talk to her on the phone in age related language to encourage her to do what she did before.
Everyone Poops (My Body Science Series) by Taro Gomi and Amanda Mayer Stinchecum (Paperback )