The HT engines clutch are quite weak and prone to slippage. These clutches are not intended to be used from a stop; however, it should not slip when trying to pedal-start the engine. Sometimes you need to adjust your clutch cable in a way that when the clutch handle is held in with the locking pin, the clutch is not completely disengaged since you could have the opposite problem being: If the clutch is completely disengaged when using the handle locking pin, it might not be fully engaged when the handle is released. I guess this kind of problem can be fixed with a disassembly and readjustment of clutch springs but in my case, I decided not to bother with this and put an elastic on my handlebar so that it can hold the clutch in when I do not want to use the engine (holding the clutch in for extended periods of time can be exhausting). In your case, try to adjust the clutch to make sure the slipping is not caused by the clutch cable.
tldr: Misadjustment of the clutch cable can lead to slipping.