If the back tyre locks up when you bump start it, are you coming down on the seat just as you let the clutch out - they don't call it 'bump' starting for nothing.
Also, are you using 1st gear or a higher gear - 2nd or 3rd is usually required.
Pull the clutch in as soon as the engine fires.
If you take the spark plug out, does the engine turn over easily for lots of revs when you operate the kickstart ?
Sounds like your ignition may not be up to snuff - try reducing the spark plug gap to 2/3 of what it should be, and see if that makes any difference. Don't leave it like that, whether it does or not, it just tells if your ignition is weak.
Engines that start easily feel easy to kick over.
You just need to sort out why...
Is your spark timing and cam timing absolutely correct, ?
Hopethishelps.