CStanford
... if you have a problem with turbulent grain another problem with these chisels, given they are almost as long as the mortise you're trying to cut, is that you're caught in a do-loop - you don't have enough wriggle room to re-register the chisel to come at the bad grain from a slightly different direction.

I think your immediate fix would be to bore out the rest with a brace or just use a firmer to finish up. I have straight sided firmers which are good for a situation like this but you can get it done with a bevel edged chisel if that's all you have.