Now knowing that its a Quad, I'm assuming it has QuadAlign which spins the part it just picked in a beam of light and a sensor measures the length, width and height of each part. If it falls outside of a defined range, it will throw it in the dump bin. On the parts it tends to throw out, I'd first make sure the tolerance is wide enough to cover part variation. If the part dimensions were inputted from simply measuring the part with a pair of calipers and the tolerance is zero or a very low value, then a slightly larger part will get rejected even though it meets the manufacturer's spec. If it picks another part and places it successfully this might be the reason for the rejects. If your old Quad does not have QuadAlign, then something else might be happening which I have little advice on and it may be time to upgrade (I don't think I'd want to run a Quad without QuadAlign).