JET 310 thicknesser feed problem

I've recently got an odd problem when using the thicknesser on my combi Jet 310. I normally keep my thicknessing light and creep up on the final size, so the depth set is usually just 1mm or so more than the timber. Over the last few days the timber is jamming on entry against the row of floating guidepins that sit in front of the first power feed roller - these pins normally lift up as the timber passes under. This jam causes the timber to kick up about 20 degrees, so does not reach the feed roller. I've checked that the rollers are not slipping and they are fine. I've also cleaned the thicknesser bed to make sure its smooth. If I adjust the thickness height to be very light - less than 1/2mm more than the timber, it will pass, but as soon as I set it to my more usual 1-2mm for I get this jam. I'm stuffed if i know whats going on? Any thoughts welcome. Thanxx