So typically stub mounting, I think. Anyway, when you see the slide stuck at bottom there must be some step inside, from wear mostly. The slides are not very solid in o.d. dimension, so when someone did some bending on slides for having less wobble at half open to compensate a worn carb , that could have been too much for the slide to plunge down into the non-wear carb bottom . A reamer would restore a nice parallel bore but then the jet holder will be sloppy in there - not acceptable as well. A new body would be perfect but unavailable from Amal today, just complete carbs. Maybe you could remove any steps you feel in the body at bottom position of slide with a scraper and abrasive.
Vic