If the packing or the valve on the bottom of the rod are faulty there will be a staging issue, not a capacity issue.

First verify with a known weight that the unit can't lift it's capacity with the hoist cyl. If it can't, try lifting the weight with the tilt back function. This will test the hoist portion of your hydraulic valve. If it will neither hoist or tilt it's rated capacity, then pressure checks before & after the control valve will be needed to find the culprit- either a faulty hyd pump or faulty control valve.