When you trim up, the bend radius of the shift cable between the gimbal housing and the bell-housing decreases (ie, the bend gets tighter). If the cable is marginal, that may be enough to make it too tight, and override the lever spring tension.
Check the cable doesn't have a kink in it anywhere. Check it's not already a bit stiff in the drive down position. Old style cable or new style? (screw-in part at the drive end threaded into the front or back of the bell housing?)
Pull the drive and check that the slide cavity in the drive is clean (like, spotless clean!) and greased. Check no crud built up in the bottom of the slide, or in the bottom of the cavity in the bell housing where the cable enters.
Chris....