Okay, this has happened with several of my neighbors, particularly those at a lower level (I am level 30)

They gift me, but when I collect their gift and go to return, I can't. The button is faded out. I go visit them, I get the bonus for being their neighbor, can play six of their habitats, but where the gift button should be is an 'invite' button. Also, they generally do not appear on my friend list.

Is this happening to others? What's the deal?!? Sometimes the next day it goes back to normal, but other times it continues, preventing me from gifting them back. So frustrating!