Are you SURE that the clutch isn't slipping?
My car did a similar thing - when I changed gear under power and dropped the clutch again, it didn't seem to "grab" instantly and took a second or two to do so.
The clutch was slipping slightly, and all it required was for the clutch cable to be loosened off slightly. Because the cable was tight, it was like you were driving with your foot resting on the clutch pedal.
Now I've adjusted the clutch cable, the wheels will spin when you change from first to second under power (especially if the road is damp)... much better
