I'm in the process of dismantling my rover v8 engine for fitting new piston rings (see previous topic on oil consumption), and i've noticed that almost half of the valve stem seals have popped off the top of the valve guides. It's random between inlet and exhaust valves. See photo (see the valve on the right!):

It's an old SD1 engine that originally had the crap umbrella type seals, but during the first rebuild I fitted the late type valve guides and proper neoprene valve seals. There is no valve retainer to seal contact going on, and it's got a stock 3.5 cam.
Any idea why they might have come off, and how I can stop them coming off in the future. Can they be loctited on for example?
Cheers,
Jeff