Push Button Diverter Leaking from Shower

My plumber just installed the fixtures in my shower/tub and the shower stays running when the tub spout is running. I can tell he didn't even test it because when I turned on the water, I could hear the air getting flushed out. Even with the tub spout running at a low flow, the shower still leaks. All parts are brand new and I highly doubt the diverter is defective. Any feedback or tips would be appreciated. I called the plumber but he hasn't gotten back to me yet.

Here's a photo before tiling. Any ideas here? I'm fearful that we'll have to cut the drywall on the other side to fix this...