As Sabrina suggested I would put May back on a leash and never tell her to come unless you can enforce it. I would forget the "come" word and use something else as she has already learned to tune that word out. Until she learns to come when called you could let her run around the yard with long cord or leash attached to her collar. I would supervise her when she is out like this in case she gets tangled up. When you want to go get her, go out and step on the cord and then you can grab it and reel her in.
As Marina suggested, turning and running in the opposite direction when you call them, works really well too. Most dogs can't resist chasing you and will soon catch up. Always give lots and lots of praise, no matter what it takes to get May to you.
Never never call May to you and then do something that they don't like, such as clip their nails etc. In this case, always go and get them instead. And last, but not least make sure that calling May to you does not always end her play time. Make a habit of calling her to you, give her a treat and let her go off and play again.
Good luck,
------------------
Jan
Markus, dark brindle
neutered male, 6 yrs old