Could be a few things.
Slipper is loose/oily and slipping too much.
Engine is tuned too rich not getting enough RPM's.
Clutch is bad/slipping.
Two speed isn't adjusted right and needs the screws backed out some.
Two speed OWB is slipping on the shaft.
Two speed shoes are hopping the flat spots on the shaft causing it to not engage.
Basically in the order of difficulty to deal with.