I am not sure if this answers your question about the stuck balance. But could it be that the coins that need to be traded into btc are off too low of value to be traded at any exchange. For example cryptsy wont let you trade 1 dvc since its its value is too low for a trade so you need a couple of coins. Since these are pools that mine multiple coins maybe the last of these get get stuck. I had the same problem when I stopped mining. I do mine it as my backup pool now with a couple of old usb dual miner sticks running on cleaver as primary.
I mine here half the time.
The other half, a pool with an API so keeping track is automatic...
I have made about 1.5 BTC, and my max possible stuck balance is about .01. (How could I know for sure?)
I database my stats, I am just fascinated with the numbers...but if the damned immature and unexchanged suddenly said ZERO, could I argue? Would it be worth my time to argue?
In my car, I keep quarters in the rubber iPad cup. The other coins I keep in an old cigarette pack (now I vape). Since 1984, all the pennies are not even made of copper...how many do I have? Forgive me Caesar, but why should I care?
BUT I am sympathetic to new miners who dont have many devices and they are trying, just like me, to catch the flow of it.
I dont care about how many BTC fall off the truck or how many Turk makes...I care how many I make.
If you arent making at least .01 per day, sell something and buy another miner...so called "stuck balances" never kept anybody from getting a payout...
Clever Mining Not-quite-a-whale-maybe...
mining it as my backup 
