Doing an oil flush only flushed some extra dollars out of your wallet.
They knew it was not going to clear up your problem, but they wanted to make sure to get into your wallet deep enough.
Never do an engine oil flush, or a tranny fluid flush. All it does is loosen up deposits that are sitting quietly in the nooks and crannies and mixes them into the fresh oil or fluid. You could flush an engine for hours and not get 50% of the sludge out, and the other 50% is loosened up to mix with the new oil or tranny fluid.
Yosemite