Thanks Andrew - no mask required in the gym, so that's not the issue; plate or bar weight discrepancy could well be an issue.
This morning was my squat-press-DL session and the press felt a lot stronger than last Friday. My hypotheses: 1). Every rack in the gym faces a mirror, no mirrors in my garden at home, lifted today with my back to the mirror, much easier to concentrate; 2). First time pressing with a belt last Friday I think constrained my hip movement, very conscious of that today, and it worked fine; 3). More distractions in the gym than my back garden - I just need to get back in the zone of lifting with a few people around - all other lifts fine, but I've always found the press to be the one lift most likely to be hit by a lack of focus.