Help- Seals are driving me mad!!

I have the old 2.5td 19j engine in my 110. The problem i have is the valve seals keep coming off hence oil down stems and smoke on start up. I have done thiis job three times, new seals each time, the last time a tried sticking them them on, but that hasn`t worked either.
Has anyone got any ideas, as to why they keep coming off and how can i keep them on. The valve stems are`nt worn as they are all new valves.

Cheers

I club seals ---, Where did you get your seals from ??? If you say there **** part then you know why .

2 lots of seals were bought from Paddocks, the last lot were Britpart packed from local 4x4 spares dealer.

Where do you suggest i get them from?