Getting your feet set is a positional issue. Tackles have more room to open up than guards or center. The movement for blocking for guard is different than tackle. If you don't think footwork matters, intentionally shorten your natural stride by 2 inches and watch how much you struggle to keep that adjustment and how much it affects everything you do.
Also, he's now going up against what were before elite level players. He could very well be overcompensating for their level of skill and power. That's why coaching is important. To teach them how to adjust to the new level of competition.