A common problem i have found is that the commutator grooves are getting full of residue from the brushes.
Some dynamo's that didn't work where ok after cleaning.
After that i clean the copper with some steel wool or scotch brite and clean it off with some petrol.
The person who made this bit explains it rather well in my opinion.
Starts at 8:20
https://www.youtube.com/watch?v=W8mSgOOzHGQ&t=701s
Regarding 'smearing' of the brushes on the dynamo commutator...Try to find some original Lucas brushes...Some of the pattern brushes are too soft and create the problem pretty quickly...You'll find original brushes on ebay....Ian
