From the manual
PUMP LEVER ADJUSTMENT
IMPORTANT: After 500 shots, and every 1,000 shots thereafter, the length of the pump lever stroke may need adjustment in order to maintain maximum performance. To check and make the needed adjustments, follow these steps:
Step 1. After firing the gun, open the pump lever and close to the point where a slight resistance can be felt and stop.
Step 2. Measure the distance between the receiver and end of the pump lever as shown in Figure 7. This dimension should be between 1 1/8 and 1 1/2 inches. If the clearance dimension on your gun is within this range, no adjustment is required.
If the clearance dimension is not within this range, proceed to Step 3.
Step 3. With the pump lever in the fully open position, place a screwdriver with a large blade between slots in the threaded portion of the pump. (Fig. 8)
To increase the pump lever clearance, rotate the screwdriver clockwise. To decrease the pump lever clearance, rotate the screwdriver counterclockwise.
NOTE: Adjusting the pump lever to a clearance dimension larger than 1 1/2 inches will overstress the gun and will result in permanent damage to the gun.
Maximum performance will be achieved by adjusting the clearance as specified in Step 2.
Also, have you tried oiling it?