Well, blocking is very tricky.

OL play is affected by many, many factors. And it is affected by rhythm--getting into and out of rhythm.

You're firing out at moving personnel. How they move can make it much easier or harder to engage and sustain a block.

Offensive balance affects OL play. So does momentum--positive or negative. It's a dynamic as old as football.

I've experienced it, and it is amazing how the degree of difficulty can shift from play to play, series to series.