I had a bit of the same problem the first time I mounted my Hollywood seat. The issue was the rear pillion, as you have noticed it needs to slide all the way back to allow the front seat to mount (and close) as it should.
The metal tab that ozzy mentioned is meant to slide into a bracket on the pillion. If you loosen the 2 bolts that attach this bracket to the pillion, you will "open up" the gap that the tab slides into. I found that this was what I had to do for it to fit. I simply removed the 2 bolts and installed a couple of washers in between in order to widen the gap a bit. I then slid the pillion back on, giving it a bit of a slap to get it all the way back there. Never had a problem since then!
Hope this helps a bit!