There are two reasons why clutches fall of or come apart un expectantly.

One scenario is where the diaphragm spring retaining circlip comes out of it's groove thus causing the diaphragm to fly out and consequently all drive is then lost.
This happens when the circlip and circlip groove become worn. The outside vertical groove face becomes worn, the circlip then edges out of it's groove until it finally lets go. Sometime a new circlip will cure the fault but, usually a new clutch basket and circlip is required.

The second scenario does actually involve the complete clutch assembly falling off. This usually occurs on Starmaker engines, older type Stormer (Y4 & Y5) engines or six speed gearboxes where the gearbox main shaft is the early type which was machined with a rather shallow circlip groove. Everything is fine until the groove and circlip wear. You should upgrade to the later deep grooved "E" clip type main shaft and E clip.
You may be able to have your old main shaft re machined, however the main shaft is made from hardened steel and it will require a tipped tool to cut the new groove. The groove root diameter should be 0.395" and the width should be 0.046"

We can supply the later type main shaft #0710191 and the later 'E' type circlip #0710190.