The trim doesn't work right unless the cable tension and limit screws are just so. If it is too loose, then when you downshift the derailleur goes all the way to the limit screw and there is nothing left for trim. I'd guess it is either the limit screw should be backed off or the cable tightened slightly.

I had to spend some time with it when I installed a new derailleur last year. Took me a few minutes of experimentation to start understanding the relationship between the limit screws, cable tension and the trim click in the shifter. Once I figured out how it all worked I was able to get it dialed in. I set the limit screws to avoid inside chain rub in low gear and outside chain rub in high gear. Once these were set it was pretty much a matter of tweaking the cable tension to get the derailleur to reach both limit screw settings and have the trim work on both rings.