When fatigue kicks in, your dominant muscle will automatically take over.
This tends to alter your swing tremendously.
Have you ever did a big shank even when you do pitching or chipping? Body tired and arms are lagging big time behind; hip might be swaying from right to left instead of rotating.
I would go back to basic and do 50-60% of my full swing, go slow and focus on keeping the body and head in the correct alignment.