Stumped?
You did not mention if you are in a no burn, seasonal burn or never burn area.
Stumps burn quite nicely if you take care of three issues.
Let your fire get real hot. If you've been feeding it for five or six hours, it's ready for clean stumps placed the fire. If the wood is dry, two or three hours may be adequate.
Clean the dirt off the roots by lifting the stumps 18 or 20 feet in the air and dropping them on a stump still attached to the earth.
Place all stumps on top of the fire, not on the side. The top may be as much as 800 degrees hotter than the side of the fire.
If you're in a no burn situation, you have to decide if tub grinding ( expensive but you get a side benefit of mulch which can be used to stabalize the site.)or transport makes the most sense. Sometimes the counties or state will accept stump fill but often it must be managed at a landfill with a dump charge.
lastly, and I am not trying to annoy you, the fact that you are having repeated trouble with stumps may indicate you are not using the appropriate tools. Often small contractors, especially SS contractors, arrive in a truck inadequate for transport of stumps or debris. Since a SS operation is flexible because it can travel on one-ton and small trailers, you, as a vendor must exercise some responsibility in accepting work.
It is possible to dig the way out of Levinworth with a spoon. If someone is willing to pay you by the hour to do things for which your machine is not adequate, God bless, but remember, when you employ a short wrench to turn a 2" nut, it goes slowly and seems difficult. If you use a larger tool, the problems often disappear or shrink.
SS machines are great and flexible beyond compare, but they are a small tool and work best when paired with light grading.
( in soft dirt, a 320 Cat excavator can load around 200 tandem loads of dirt a day and over 125 in heavy dirts and shale or light rock. It would take 10 or 12 big SS to do that ( in soft soils) at a cost of 5 to 6 times MORE! In heavy and rocky soils the SS may be useless.
On larger jobs, I would suggest finding a larger contractor who can do the front end clearing, transportation and then take over after the heavy trash is managed. You may be surprised to discover more money in your pocket and fewer machine repairs by using this tactic.
On smaller clearing jobs you may want to anticipate transport or grinding in your estimate. It is costly to stall a crew because trash management is underbid.
If you haul logs to sawmills and only have to manage tops and stumps, obviously, there are fewer cubic yards of trash to manage and you have created a cash flow from the trash. Remember, it takes significant wood to burn stumps, so if you're transporting logs to saw mills, make sure you keep some logs to burn with the stumps if your tops prove inadequate.