First, I would ask which object and/or pet are you using which has a maycast that isn't working? And what is the spell in question?
That being said, I have items and I have pets with maycast spells on them, and the maycast's do work - or, at least, they have worked for me, but I've noticed the maycast is a rare instance. It isn't at all common in my own personal experience - but that depends on the item and/or pet. I have pets with maycast spells such as maycast sprite and/or unicorns who tend to spam the spells a lot more than the weapon (a spear) I have with Queen Calypso on it, and that casts more rarely - so it could have something to do with the spell itself.