To the OP. your concerns are exactly what this forum has been helping me with on my thread, "diary of a Surdiac". It's been here for almost a year and a poster above has even mentioned it. Have you looked at it yet? I won't bore everyone with what's already been run thru the mud. But I will comment on a couple things.
1 Your stove, (or any others) don't magically drop the ashes into the pan, you have to work at it. Some more than others. slicing the grates is only partially effective. you also need to poke from "under" the grates.
2 A reply above slammed the performance of these stoves, and I can't say it's all totally exaggerated, and I'ld be one to stand in line, and give these stoves a 1 finger salute. But mainly because, all this free standing stove ordeal is new to me. I'm betting my problems, (and yours) isn't the stove, but the operator. I didn't originally buy the stove, for coal burning, but to make a long story short, it will now do "mostly" what I wanted it to do. My house has an oil heating system, and is a 2 story, only 1600 sq/ft. This stove, (my 513 is a step down from your 713), is installed off my living room, and will heat both floors comfortably. (way over 75* easily when the temps outside are in the teens.) It is used for supplemental heat in the extreme cold snaps that hit us frequently here in southern New England. not a stove I would use in a 3000sq/ft Maine home, needing heat 4-5 solid months out of the year.
My complaint (like yours) is trying to get the burn times longer. I've had it go 8-10 hrs unattended on occasions, but not consistently. That's what I'm honing in on. this forum has been a wealth of help towards my (and your) dilemma, so join the fun, and pick up where we've left off on the other thread.