I've seen two different designs.
One, the spring that retains the bolt is at the farthest back part of the rail. Insert bolt, rotate head, pull bolt back so it is tight back and press the bolt head down until you feel it click. Slide forward.
PROBLEM: Some of those retaining springs build up loads of crud under them. If it looks dirty in there, remove the stock and flush out and clean under that spring. Little bits of grit and dried grease/cosmo are really good at jamming that up.
There may be a gap in the rail further forward. Saw this ones a long time ago, don't remember much except the bolt head traveled to that point, then rotated over there.