As someone pointed out having van Dijk, a player that can be dominant in the air even against big and strong PL players probably makes some difference.
Vertonghen and Alderweireld are fairly strong, fairly big, but they're not dominant in the same way. I think the same can be said about Dier and Kane. Sanchez seems to me to have very little accuracy with his headers all over the pitch, no surprise that he struggles to finish headers even when he gets to them.
Then the set piece takers. We haven't prioritized bringing in a specialist for that.
Despite this we were doing fine last season. This season everything has been a mess, no surprise set pieces are included.