The amateur produces when he feels like it.
As a diligent person, he might have produced a list of potential things to write about. He'll look at this list when he fancies writing.
But when he fancies writing, if the potential writing topics on the list are unexciting, he'll probably leave writing to later.
That doesn't mean that he'll never produce anything. Just that he'll be guided by his present emotions. Novelty guides him, and he waits for inspiration to strike before writing.
Sometimes, even when inspiration strikes, he'll delay producing his thoughts because it's not quite the right time. The inspirational idea is so good that he'll save it for later. At that point, he’ll have more of an audience, more time to write the idea fully, or otherwise better able to execute the idea.
By not consistently producing, his ability to produce – even when inspiration strikes – is rusty.
He's like a runner waiting for great conditions to run his best time. He might run when those great conditions arrive, but he'll be slow.
He's unable to seize the moment because he hasn't been practicing during all the times when the conditions weren't great, and when he didn't feel like it.
Consistency trumps intensity.