After having two prints fail (one of which was 3 and a half days wasted!), I discovered the reason why.
The slicer creates a primer line :
This line, once it gets to the end of the belt, can get trapped here :
This results in enough tension between the trapped piece still connected to the end of the print to pull it off the belt.
Could save you some time and plastic knowing that 
Which slicer are you using?
IdeaMaker. There isnt a lot of choice for printmills. And you really need that purge line to start the print, as the head is at 45° so you dont get brilliant adhesion. I use hairspray to help.
Kiri:Moto also slices for belt printers. I don’t know whether @stewart has addressed that particular problem (making the primer line a few layers thicker? I dunno) but he’s certainly built another slicer that supports belt printers.
Yep, KM has a lot of belt-specific features. Two that are really important are leading anchors and side brims. You can also change most settings on a per-layer or per-layer-range basis. So you can over-extrude the leading-edge, for example. The leading edge anchor can be turned into a wedge shape so that it gets knocked off by the scraper instead of pulled around the roller causing later print failures. This is a feature unique to KM.
Thats great - I will give it a go.
