Robert,
The R clips sit in a little groove in the tube that is attached to the seat pad so that the arm closest to the pad sits within the tube - see pic 2. The rods from the seat that the tube slides over have three notches equally spaced that I believe the R clip arm will nestle in when you slide the pad fore and aft, which as mentioned provides the adjustment settings.
So the R clips are fixed and the pad should slide up and down the rod.
If your pad won't slide back I can only imagine it is just a bit stuck and needs a little extra force, mine adjust with ease.
if you are able to prise the R clips off then that may help you get the pad off to see what might be going on.
Pics below to help explain. Let me know if I can be of more help.
a seat 1 by snapper seven, on Flickr
a seat 3 by snapper seven, on Flickr
Cheers
David