Can't get timing dialed back to tdc

Well, consider this ; suppose the timing mark slipped 10 degrees over a few weeks or months, or years,for the previous owner. So it started to run worse and and worse. So the previous owner gives it a tune up, including resetting the timing . So now everything is lined up again, but the timing is all wrong. The rotor lines up,just as it should..
So to your first question, checking the TDC mark. You need a special tool called a piston stop. It screws into the sparkplug hole and when you turn the crank by hand, the piston comes up close to the top and hits it and then stops.Then you turn the engine backwards until it again stops.The exact difference between the stopping points, devided by two, is true TDC. The stop is usually adjustable to fit multiple engines. So what I do,on a slanty is use no 6 hole, so I don't have to mess with the alternator.I take out all the plugs to make hand-turning it easier Than I line up the TDC ballancer mark to the TDC pointer. Then I turn the engine over backwards to 10* advanced. Then I install the tool screwing it in until it hits the piston, and lock it down with the jam-nut. Then I turn the engine a couple of degrees backwards. Then gently slam it into the stop. Mark the ballancer where the TDC index tab says TDC. Then rotate the engine over backwards until the piston again gently slams into the stop, and again mark the TDC point on the balancer. Remove the stop. Crank the engine over a few degrees until both marks you made are clearly visible. TDC will be exactly half way between those two marks. Measure with a flexible tape, and mark it. That is true TDC. And you have proven it.
It may well be that your new mark is exactly where the factory put it.But until you have proven it, tuning a problem like yours is all guesswork.And if it is not within a degree or three, it is junk!
Good luck