What is causing motorcycle slipping in 1st gear?

Your motorcycles gearbox is a complex, balanced piece of equipment, and works in tune with the motor. It sounds like your first gear might be off kilter or "stripped" or perhaps you need to seriously overhaul something. My advice is to be gentle while in first gear and then shift into second and see if you can feel something in the shift pedal/knob that might give you a clue as to whats wrong.