Seán Murphy — should I charge ad retainer or commission?

I've been doing sponsored content and ads for two clients on a flat monthly fee, ten hours a week all in. They're happy, but the scope keeps creeping and I'm losing my evenings doing revisions nobody asked for. My follow-up on new avenues is weak too, so I'm not chasing more of the same. I keep wondering if a commission model would let me stop chasing and just do the work.