Not looking to hijack this thread but I want to jump in with similar issues/problems.
I installed an AX15 with an external slave cylinder. I have the same issue when shifting into 1st at a stop. When I first start driving, it's "notchy" and stiff to shift into 1st and 2nd. It is worse when I have drive a few miles. I have to shift to 4th, then 2nd (hard), and then to 1st (really hard). I still have really push it to get it into 1st. There is no grinding or other noises coming from the trans. If the truck is slowly moving, downshifts to 2nd are fine.
I thought it might be the hydraulic system so I replace the master/slave with a pre-bled system. There was no noticeable change.
Since there is no grinding into gear, I am thinking the syncros are fine. (I may be wrong.)
Can there be an issue with the detent springs and balls? If these are worn out, could they prevent the shift rails from lining up properly? (Just throwing "stuff" at the wall.)
Problems with the pressure plate and disk?