I've experienced the same situation a few times. When I first installed my stove in early November, it would burn great for a few days, then heat output would drop off and I almost lost the fire one morning.
I learned from that experience to do more poking through the bed of coal ,(as Devil has suggested in other posts, ) as well as shaking more aggressively. I usually continue shaking after I see some red embers fall. Then I poke down through the top loader door, then shake a few more times. This is usually sufficient, but some times I have to repeat this procedure a second time. I always have the ash door open and fire livened up before I start.
Last Sunday, 12/28, when it was 55 degrees, I let my fire go out and cleaned out the stove for the first time since early November.
One thing I really noticed after it was burning again, was that when all the coal in the stove was fresh, and hadn't established the ash /red burning embers/ black unburned layers yet, it really put out more heat. Normally, I can put my hand on the bottom plate and ash door of my stove. But with a fresh fire and full bed of new coal, the bottom was quite hot to touch.
Yes, you want some ash to insulate your grates from excess heat, but at some point, it's too much.
Sometimes, you have to poke and prod the fire a bit!