DS2 is 16 weeks (4 months next week) and has rolled both from stomach to back and back to stomach several times. One week he did both many times. Now we can't get him to roll either way. What is up with that?

Other notes: He does grab his toes and roll onto his side. He hates tummy time and screams after about a minute.

Is it a problem that he WAS rolling but now isn't?