Breville Bar Italia reduced to 3 cup over time?

When I first used it, it would make 4 cups (one jug) within about 7- 8 minutes. Now it only makes 3. I have cleaned it as recommended and also tried flushing with vinegar to clean any lime deposits, but still only 3 cups come out, then nothing but steam instead of the 4th cup.

I release the steam pressure, add about 50% more water, wait for it to boil and presto, I get the 4th cup.

Why has it reduced it's output, and can it be fixed?

It is used 3 - 4 times a day and is about 3 months old.