I had a very similar issue with the shim on the first one I assembled (see here which didn’t get any response: Pi onoff shim loses power after a few seconds )
I assumed it was a soldering issue and purchased another one to try again (which i ended up destroying along with a pi zero w as I forgot to unplug power when i tried tidying up one of the solder joints on the header with the soldering iron = kzzzrt pop!)
however recently I looked at the first one again and I noticed one of the tiny board mounted components was actually loose on one side and had been twisted round 45 degrees. so that one ended up going in the bin as well.
third time turned out to be the charm and finally got one working when I assembled my project on a black hat hack3r pcb with a cut down male header with just the shim pins needed and only soldered onto the pins required to make it work.
now it works flawlessly but i get the feeling it was more luck than anything else as it’s quite a tricky thing to install as soldering to header pins is not easy to do well for beginners.