That’s surprising. If everything is fitted I would expect to hear chattering. Spinning freely normally happens when the chain has been removed which some people do when the clutch fails. The only other things that come to mind are a bit more serious.
Firstly it might be the large gear that engages with the starter clutch is either a wrong one or the boss has been machined down so it will not engage with the rollers. The diameter of the boss should be 42.2mm. I would guess that it should be no less than 41mm. If that is fine then we need to look deeper.
It may be that the whole of the rotor assembly is rotating on the crank shaft. When fastened up correctly the taper of the rotor mates and secures to the taper of the crankshaft. Obviously the tapers are very important so if one was different to the other then they would not mate and it could slip. The B1 and B2 have different tapers to all other models and are NOT interchangeable. This is because the B1 and B2 are 3 phase charging whereas all other models are single phase so a different taper was used. It is possible that the crankshaft or the rotor have been changed at some point. Maybe this is why the bike got laid up? You should be able to tell that as the rotor would ‘wobble’ before tightening the centre screw, quite an easy check. I would also suggest check all the components are there, in good order and fitted correctly.
I hope this helps.