What I usually do is:

1. Exit PL
2. Go to AppStore / iTunes in phone and sign out and sign in with account.
3. Sign out and in with my iTunes in PC - plug in my phone.
4. Log in PL.

Fixes everytime, I've had this happen to me 3 times, one on new phone, new ipad, and I accidentally deleted PL off my phone and had to put back in.