Force should not fall off at all, because the actuation arms which extend into the lens and transfer the motion from the body to the cam plate, move in a fixed arc, regardless of what point along those arms the cam plate makes contact with them.

A few things to consider: The body does not generate much force on release. The pins must have only a little resistance to being moved at the start of their travel. Cocking the lens requires some force, which provides the spring tension to work the shutter and diaphragm, but once cocked, the pins can be moved in the release direction easily.

Check manually that the pins move easily in the release direction (clockwise, looking at the back of the lens). To release, move in the lock tab located on the lens, inside the mount ring between 4 and 5 o'clock as you look at the back of the lens. The pins will have a slight resistance initially, then move very easily until they reach a detent near the end of their arc.

Check that the rotating piece which is attached to the back plate, and has the pins and actuation arms on it, can move completely freely through its entire travel.
Check that you put those actuation arms into the proper slots in the cam plate (the plate which moves the rollers as it it is turned).
Check that you used the right grease. A light grease which is not sticky should be used, as a thick and/or sticky grease will create resistance to motion.
Check if anything seems to be misaligned and feel if anything seems to be binding.

The critical point is where the rollers contact the cam plate. That is where a lot of localized force is generated, and all the old grease must be gone. The cam plate must move easily in the release direction, and the rollers must follow.