This is a common complaint about pneumatic squeezers and the answer is always the same: "Widen the gap between the rivet sets."
Why? The pneumatic squeezer, or sometimes called a compression riveter, uses a wedge and cam to develop power to squeeze the rivet. When the throttle is activated the initial movement of the cam causes the ram to move quickly. About 2/3 of the way the wedge angle changes to provide a considerable increase to the leverage to squeeze the rivet.
If the gap is too narrow, the wedge will not reach the second angle and the squeezer will not have enough power to squeeze the rivet. This can be as small amount as a "hair" more. By widening the gap between the rivet sets the squeezer reaches the high pressure portion and continues to squeeze the rivet.
If this cannot be achieved, the next step would be to change the yoke to a wider yoke, or use a shorter rivet.
The set up is critical on any pneumatic squeezer to achieve proper pressure for riveting.