I find it VERY hard to do this. ( I just tried it. Turn wifi on in camera; open WMU on iPhone; [make sure there's a jpeg copy of the photo as it won't download RAW apparently]; flip back and forth on the phone numerous times cursing under your breath and maybe it will eventually transfer.) Consequently, I don't do it very frequently. It's easier just to go home and download to the computer and airdrop selected photos to my phone.
Once wifi is enabled on the camera, you need to connect to the camera wifi network before you open the app.
Are you doing this ?
Dougie.