2007 scrambler 500 pops out of gear FIX?

I have a 2007 scrambler 500. had noises in the Tranny, pulled it to find the Needle Bearings where Scattered bottom of the input shaft high/reverse. also the silent chain stretched. Replaced both. Cleaned and Inspected every gear, tooth and chain inside All Looked very Good. Put All back together back in the Machine. and it now Pops out of Forward. Reverse is fine? pops out going slow or hard within 100 FT. So is it A Worn Shift Shaft or Shift Fork. or something I didn`t do Right?
Thanks for your Help