I wouldn't say that, since, if he were ever able to do that, then he wouldn't be surprised that King can.
More likely that writers have difference processes and King and Martin are entirely different.
Yeah, King writes a fuckton, but there was also a 6 year gap between Wastelands and Wizard and glass, and then another 6 year gap between wizard and glass and the wolves of calla. That's the only thing even comparable to the scope and size of Martin's books that King has done, and he stalled as well, then finished the last three books within a couple years.
You can't compare like to like with art. It's fucking art. Yeah, it's likely that Martin is stuck, but if that's part of his process, then that's what it is, and, as a writer, I can respect that.