This image is for a KA but it's applicable to the CA18
Should be two springs? One holds the throw-out bearing deal to the fork. The other holds the clutch fork to the pivot on the bellhousing. It's likely that that spring is bad and giving you problems. Not holding it on there good enough? I dunno. I bought new ones when I got the new clutch. The dust boot sorta supports it too, in a way.
Also, if the pivot itself is worn down, the retainer spring might not "grip" it well.
Honestly, just buy a new dust boot + pivot + springs from
http://www.courtesyparts.com. They're cheap. KA stuff should work. At least I think so, pretty sure that's what I'm running on mine. But it has been like a year since I messed with any clutch stuff, haha.
Other than a worn out retainer spring/pivot I can't think of any other reason the fork would be falling off. Maybe triple check everything and use the new stuff? Can't hurt.