Musescore isn't seeing that a bar is full and is also leaving me with unwanted rests. Beginner!

Hi almost total beginner here. I managed to complete one effort with 12 bars to my satisfaction but after a couple of weeks away from using Musescore I seem to be stuck with Musescore misbehaving or else I'm doing something silly.

I've set up my new effort in 4/4 time but Musecore isn't moving on automatically when the first bar is full. It keeps accepting more notes. If I click in the next bar myself then I'm left with rests in the first bar that I don't want.

I really don't understand how I managed with my previous effort!