Warmer air at A/C ducts

[Stocker]

Got my A/C working again for the first time in many years. Converted to R134a, new parts installed by a local mechanic (don't shoot me). Got a problem we can't figure out, though. I have searched a ton of threads and can't find a solution.

It's blowing lots of air but it's quite a bit warmer at the vents than at the bottom of the HVAC airbox. In my driveway around noon, ambient temp nearly 90 degrees. Had 33 degrees at the bottom of the airbox (great, right?) but 62 at all three A/C vents. Ran it for a good long time, mostly shaded by a tree, no difference. Ducting is new, undamaged, and properly connected.

I'm at the end of my rope and need a miracle. Thanks for any help.