I had that and 90% it was air drafts and just being too cold in the room, The bed is only heated in the middle and the bed temp can be quite different around the edges where it can radiate the heat easier. The only time I’ve had trouble recently is with parts that have a lot of surface area contacting the bed and hairspray seems to have cured that. most of the time the parts are stuck down pretty well but after letting the plate cool the parts are often loose and can slide off.