Boner down

My boyfriend & I have been together for 9 months.

The first 3/4 months we were together, we had sex atleast 5 days a week.

Now, we maybe have sex once a week...

Last night he went down on me and usually after that, we have sex. When he went to put it in, he got soft and I tried to go down on him but nothing happened... what does this mean?