Invisalign Doesn't Stay on Back Tooth - Is This Okay?

It should be okay if the back tooth isn't fitting properly, as long as the other teeth fit well. I would continue to bite down on it and once the material softens a little, the fit should be better.  The movement from invisalign is more likely occurring on your front teeth so it is okay if the fit isn't perfect in the back. It is more annoying to you than anything else, but should not impede the tooth movement process.