Had this exact thing on mine after the ititial setup, swear it got worse after about a month of use. Got my own 8" Starrett level and slate was off over 3 lines in areas, 2 to 3 lines in most. Over half the leveling wedges, especially the center wedges weren't even touching the slate, so slate was mostly low in the center. Also my cloth stretch was really horrible and twisted, and intensified the off roll in one corner. To top it off, balls slow rolled past the center pockets, would move out then back in.
First thing I did was to adjust all the wedge bolts by hand till the wedges just touched the slates. Then every week or so, I started raising the center wedges 2 bolt head flats a week. The reason I didn't just bring it up to level all at once was I was really afraid of breaking the superglue joint on the slates if I went too much at once. Took months to get to a reasonable level. Nothing I can do about the bad stretch, and that will half to wait another year or so till I have it recovered. Where a slow roll cuuld be off up to 3/4"+ on the original setup, I am now down to about an 1/16"-3/16" on all corners except one, where the cloth stretch curve pulls it out more to about 1/4". The level did sink to the center about another line on the level after another year or so, and I have since adjusted it back. A one piece slate would be simple, could just do it all at one time.
Diamond has a great video on how to level, although there is a couple of things they don't discuss. One how to back off leveling bolts, quite simple if you think about it. Then there is something they casually mention, without explaining, and that is having the top on when leveling. They say to have it on, but don't mention bolted down or loose. I just loosened the rails in the areas that were affected by my leveling. Not much of an issue when bringing up the center, but I had one end slate that was up twords the center, but down 3 lines on the outer edge. I was a little concerned when I realized that raising the outer wedges was actually trying to pull the slate joint apart.
While the framework on my professional is massive, it is still wood, and from what I have seen, will move some over time, even though it's slightly. Would not be noticed without a high quality level.
My table is in a heated and air-conditioned space.