The trick I've used many times on my 6V motorcycle dynamos to test them is:-
Take the two wires that come out the dynamo F&D, connect them together and connect them to the live end of a single filament bulb (12V is best for this purpose) Earth the cap of the bulb to the dynamo with crocodile clips and run the engine. The bulb should light up and glow brighter as you rev it up. If it doesn't, As already suggested, check/replace the brushes, clean up the end of the commutator with thinners/switch cleaner etc. The build up of carbon dust and shite can insulate the commutator and or cause the brushes to stick up in their holders.
By the same system you can test/motor the dynamo on the vice by connecting those two same joined wires to the poss of a battery and earth the body to the neg side. The dynamo should motor in the same direction that it charges at.
I've said an awful lot here for what is a very simple test operation. Ron
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