It is not something like a hosepipe or cable fouling on the gearbox linkage? A quick check would be to get someone at the helm to move the lever from Fwd to Rev while you watch the gearbox mounted linkage.

Failing that, I found the following which may help you:

Hard or Stiff Operation
The key to determining why a control system is not operating properly is to isolate the individual parts of the system. By the
process of elimination, you can usually rule out the control unit itself if only the throttle or only the shifting is hard or
1. Both throttle and shift equally hard to operate:
First, look for visible signs of corrosion. If the bearings in the control unit are stiff, it will be very
difficult to pull the handle out into the ‘throttle only’”position, and it would resist springing back in. If this is the case,
disconnect the cable clevises from their levers. Try spraying a thin, penetrating type spray into the bearings, while working
the handle back and forth. If the control remains stiff, it may require maintenance.
2. Only throttle or only shifting is hard to operate:
Disconnect the suspect cable at the control and at the engine end. Cycle the cable with your fingertips to ensure it moves freely and with no binding throughout its travel.
The cable should move in and out easily by hand with little or no friction.
If cable binds, or is stiff throughout its travel, replace it. The length is typically ‘hot stamped’ in the plastic jacket on one
end, and may be in inches or feet. The measurement is tip to tip.
High Efficiency NW Controls Model 95 cables or equivalent are required
for proper operation of engine controls.
Engine cables are subject to deterioration from the high heat in the engine room, condensation, salt water, high usage,
heavy loads, damage, etc. They have a typical life-span of 5 to 7 years and should be replaced if corroded, stiff, binding,
kinked or if the jacket is cracked or peeling. They are not internally serviceable.