Normally, a clutch will start slipping in the higher gears. Lower gears require much less torque transmitted thru the clutch to get things going. Not saying you don't have an issue with the throwout bearing, but a worn fork will normally cause the clutch NOT to release completely while shifting gears or actually make it impossible to get into gear, especially the lower ratio gears like 1st and reverse.
Slipping in lower gears and not in the higher gears would be very unusual.
