The other simple explanation is that Mummy = boring routine/a little stricter than grandparents / not as much attention/fun stuff.

Olivia was a half day with my Mum about once a fortnight. She gets one on one attention and a gadzillion exciting treats (like cooking with granny, using textas without Charlie interfering, without Mummy nagging about pack-away etc etc, LOL!)

Often she is not at all keen when I go to collect her for the simple reason that Granny spoils her and I don't LOL!

Good luck!