Driveshaft spline chamber venting ?

[Ron R]

.....since there is very little air getting in to release the suction/pressure then it is a firm push in with the grease plug out and a firm pull out as well.

at least this is my experience...

Yep, that's my experience as well. Without the grease nipple removed you won't be able te get it in or out.

Another limiting factor can be the temperature. When cold the grease is firmer than during a hot summerday