I ran into this problem a few months ago and am still feeling the side effects. Even commissions with interesting subjects had me dragging my feet. What gives?
For me, it stemmed from art turning too much into a job. I had been doing commissions for so long that the bulk of my work was for other people. This isn't bad in itself; I certainly needed the money and my clients always gave me cool stuff to play with. But art had become "what can I get out of this" instead of "how can I make this really awesome?"
I broke away from it by going on commission hiatus and drawing personal art, which sometimes means just dumb adoptables, but hey it's loose and fun and has no pressure. It was sort of amazing. After months of laboring each pen stroke, the moment I put commissions on hold I was pouring out stuff and experimenting again.
What about you?