I'm only replying as you haven't had much feedback, as heavens knows I'm no expert. I don't have an ATV type flail mower, but do have a 5 foot flail that I use on my 37HP tractor. Long grass is perhaps the easiest to "bog down" on. I can happily smash up quite heavy stuff that's dry (accidentally ran over a fence post the other day and now have a pile of chippings instead), but have to be a bit careful with long grass. My flail is the "heavy duty" type with the hammers, not the Y shaped lighter flail ends (if that makes sense?). What I think happens is that the grass doesn't exit cleanly from the back if I don't set it up just right and recirculates as a mush slowing down the hammers. What I do is set the skids at the highest setting and make sure that I have the angle of the mower set up so that it ejects the grass more easily. In my case (from trial and error) that means very slightly tilting it up on the 3pt linkage. I also make 100% certain that there is no dried debris on the roller / exit assembly. Anyway, when I do all that the grass shoots out the back and I don't get bogged down (much). Of course, I always flail when possible when it's as dry as possible, lots of soggy stuff always means that you have to run slowly and uses a lot of diesel. Another 15HP would be very useful sometimes!
Sorry if that doesn't help, I'm sure that all mowers are a bit different in design and how you set them up.