Often while riding my scooter I get a feel that my rear tyre has gone flat as I experience sudden loss of pick up and can sense the engine is working hard and poor handling(all sort of signs of a flat tyre) but when i stop and have a look it's all ok. I got a flat tyre recently and had to replace the tube ,I got it inspected by the mechanic he said the wheel and tyre condition is all ok but still I keep feeling the same sometimes. NOTE: I always maintain the recommended tyre pressure.
Normally if the engine seems to be working or revving too high without driving the vehicle forward its an indication that there could be an issue with the clutch. If you're not sure this will need to be determined by your mechanic who should be able to inform you far better of the cause of the issue. If the bike has a manual gearbox perhaps something like a clutch cable adjustment is necessary, however if its an automatic different checks will need to be made.