How to troubleshoot Canon Speedlites that are not firing at the same time?

I’m working on some studio lighting and want to have my two Canon Speedlite 580EX IIs fire simultaneously using Canon’s built-in wireless communications (i.e., I want the master to trigger the slave, but also provide some fill flash). I am pretty sure I had this working at some point, but now I have noticed that the master flash fires when the shutter is pressed, but the slave flash fires a good 1 or 2 seconds after the master has fired (and after the shutter has closed and photo has already been taken). I have been poring over documentation and videos wondering what I could have missed. Surely it is a setting somewhere that I have accidentally set?

MASTER 580EXII
Set to ETTL
Channel 1
On
Ratio A:B is 1:1

SLAVE 580EXII
Channel 1
On
Group B

Notes: I have tried a couple of things:

1. Connecting the master and slave with the mini phone jack.

2. Making sure the master is directly in front of the slave’s panel so there could be no interference with the signal.

3. I have tried it with the Master ON and the Master OFF.

4. I have tried it with the Master both on the camera’s hotshoe and on a jotshoe adapter.

5. Even tried replacing all the batteries in both flashes.

6. Have tried both flashes in Group A.

I will try to attach pics of the LCD panels. I must be missing something obvious… please forgive me if I did. Thank you in advance!

P.S. I have the Master set for a -1 FEC but I have tried removing that as well.