Remove the slipper entirely and start over. First install back pressure plate, make sure it sits properly on shaft, then disk and spur. Make sure the disk stays inside the groves of spur. Install front disk and pressure plate. Install washer, spring and washer (I think that's the sequence) then lock nut. Turn lock nut till tight, do not over tighten. Back out a full turn and half.
Turn radio and car on. Hold car down with one hand and pull trigger to check slip. If there is no slip, loosen still 1/4 turn. If it slips a lot, tighten 1/4. Then test car on carpet without holding it.
You want the slipper to slip for 2 to 3 seconds when you pull the trigger to full throttle.