I never used coaster brakes and don't know anything about it. I was curious and watched this RJ bike guy video for single speed coaster brake hub:
I would imagine you're getting problems with the clutch engaging when backpedaling. Some stuff is either blocking the teeth from contacting, or the driver is loose. Or the brake pad is worn and loose. But the i-motion will have different mechanisms and you'll have to look at the manual to figure it out as you disassemble it. I also would imagine that the braking creates a lot of metal dust if the i-motion brakes work similarly to the hub that's in the video. But I could be wrong. You'll only find out if you disassemble it.
You can find some photos of the i-motion 3 disassembled here:
http://www.rideyourbike.com/sramIGH.shtml
I've disassembled my old SRAM Dual Drive II several times in the past, and it looks surprisingly similar to the i-motion 3 except that the Dual Drive has a freehub and no brakes, but I can't figure out how the brakes on the i-motion 3 works just by looking at the photo. The Dual Drive gets very dirty from wear and old grease, so I used to clean it out and regrease/oil once every year or 2. Following the instructions the first time was confusing but easier on the 2nd time. One of the main things, assuming the construction is similar to the i-motion 3, is that you release the left nut first and most of the internals then freely pull out from the right side. In reverse, you assemble all the internals onto the axle, then slip it into the hub shell from the right side.
Work on a very large table so you can lay everything out flat. And I didn't take photos but it would have helped during my first time if I had an image of every step of disassembly to review when I forget how to put it back together.