If you are not getting enough ashes to fill a small ash pan after burning 125 pounds of coal, I suspect you are not shaking the grates enough, and are getting a build up of ash under the fire. This will cut down on burn rate and heat output. If your flue temp is 290* and still not putting out a lot of heat, that would suggest that you have too much flue draft, and are loosing too much heat up the chimney. You said the flue was 75% open right? That must mean you have a manual damper. If so, try closing it to 50%, or better yet buy a Field controlls type RC barometric damper, and set it to "4" . That would be .04 inches of water columb draft. That will keep you from sucking too much of your heat up the chimney. Also, try setting a fan directed at the stove. If you can just barely touch the stove, it is putting out about as much heat as my stove does when I set it for a low burn on a warmer day. Try setting your air intake draft to about 50% open. Too much air intake can actually cool a lazy fire, whereas is will make a hot fire hotter.
The fact that your are keeping the fire going puts you ahead of many other beginning coal burners. Even though you have some lump coal already, I recommend you buy a few bags of brand name (such as Blashak or Reading) nut size coal, and see if that gives you a better burn. Don't give up
Learning to burn coal well takes time. I learned much of what I do know from the coal veterans on this site. It seams I learn something new every week that I burn coal, which so far has been just under one year.