Hi guys,
This sounds like a typical mag problem to me, I even had it on my Scrambler, (which by the way starts very easily now, If you have a strong leg - 777cc and 11:1)
When the mag is timed internally to the points it needs to be assembled so that the points are just opening about a nano second after the point of maximum flux, Flux is the point that the magnets have generated their max voltage in the coil, This is a little funny as it needs to be at two places at the same time with a manual advance. Advanced and retarded, get this wrong and the spark will be too weak at one place or the other. Especially if the mag is a little tired.
Easiest answer is to give it to a good mag guy to check it over and tell (ask) him to make sure it sparks correctly at both full advance and retard. And test it by hand when you collect it. Or put it on the bench and mess with it until it works. That's what I did and it's lovely now.
Out of interest, after my initial struggle I fitted a brand new BTH batteryless K2F type magneto and this would not run the bike at all, Tiny little spark and nothing, only a very occasional pop. The spark was so weak and small there was no way in my opinion it was going to run my engine. 700 quid wasted. The original K2F Wader mag now works very well.
In essence, If you are strong enough and kick it with a good lot of body weight behind you, and kick it right through the full stroke from Just past TDC then you can start anything. High compression, twin carbs, and fast cams.
Just also check that the bike is not wet sumped, this makes for one hell of a diference in kicking power needed, and therefore harder also for the engine to "catch"
Good luck,
Julian