If find getting saddle rails onto the nylon seat mounting, takes a bit of a ‘Knack’. I find its best to put one rail in the slot on one side of the seat mounting, and then rotate the saddle down so the other rail ramps up and then snaps into the slot on the other side. Sometimes I have used a screwdriver to carefully lever this 2nd rail into place.
Once in the slots the saddle can be adjusted as far back as you like, and then re-fit the clamps . It is best to keep the clamp bolt as loose as possible - just enough tightening to close any gaps, but not enough to put the seat mounting into permanent compression - which could weaken it.