DaveM is correct. You probably need more rear ride height.
Check your rear sag. Measure from some fixed point to the ground while lifting up the rear so the shock is at the top of its travel. Then repeat the measurement from the same point to the ground with you one the bike. (You'll need a helper).
The TA has about 7 in of rear travel and I'm betting you're using about 3 in. or maybe more with you on the bike. My TA (when it was stock) liked just less than 1 1/2 in.
I'm betting this will solve your problem. If the stock spring is sacked, you can build a fairly cheap spacer from many different materials. Make the spacer about 1 in. thick and dial up preload from there.
As an alternate, you can drop the triple clamps on the fork tubes a bit being careful that you don't smack the rear fender on the crashbars or bottom clamp at full compression.