The shifter knob button gets dry and that
makes it hard to push, I have replaced my
knob (T handle head) 3 times, and have found
this to be a problem.
My button keeps losing the retainer tab so it
falls out, I put a little thick grease in the hole
and put the button back in (tired of changing
a defective part) and this both keeps the button
from falling out when I'm wheeling and makes it
much easier to push in.
You might also want to check the adjustment,
the knob head rotates, this is the adjustment,
if you ever take the knob off, be sure to count
how many times it spins to get it off.
If it is too far up on the shaft, the pin won't clear
the rack/lock thing in the base of the shifter, spin
on farther to get more clearance.
Good Luck
Wild