Some of the topics covered so far are things I have experienced with a spoilboard that just sits on a plastic grid.A the machine is sited only a hundred yards or so from a river,with weather that goes from quite warm to quite chilly,we have found that covering the spoilboard when not running the machine is a big help.Otherwise the top surface expands or contracts,the sheet bows, and the lack of contact with the table makes it hard to get a seal.The other thing that happens is that the Z datum moves up and down which necessitates establishing it each day if the depths are critical.
When the OP mentions that holding a full sheet isn't a problem,does that indicate that the table doesn't have zones that can be turned off? If so,the old standby of a piece of plastic over the unoccupied area can improve things.
Pockets coming out undersize is something that almost certainly has nothing to do with the spoilboard.In no particular order I would check tool size,wear on the Z axis or wear on the spindle bearings.Tool size is easy,just cut a groove and measure it to see what the tool is actually cutting.For worn components,cut a square of the same size-once with a conventional cut and once with a climb cut and compare the two.