The smaller of the 2 release levers controls the latch release for the center post.There is an adjustment on the cable at the lever end.I did mine by trial and error until I got the latch to release fully.
If the little retaining clip pops off,pulling on the lever won't move the latch at all.If you can get the pin to pop up as you describe,use a small flat blade screwdriver to push the latch aside.Then pop the cover up.You can see the cable going to the center latch.Pull on the lever to see if you get cable/latch movement.
If the cable is frozen,replacements cost 10-15$.You could try spraying it with lube but mine broke despite lubrication.I chalked it up to age(my car is a 1974 model)