Move the master off-camera to a spot where the slaves can see it.
Canon’s built-in wireless system is quite capable and reliable–provided that two conditions are met. First, the slaves have to be able to see the master Speedlite directly. Second, the slaves cannot be pointed towards the sun as they cannot see into the sun any better than we can.
Above left: The on-camera master cannot cover both slave Speedlites. The master Speedlite has a limited angle that it can illuminate. The slave on camera-left is not firing because it did not see the pre-flash instructions coming from the on-camera master.
Below left: Moving the master off-camera on the OCF33C cord enables it to cover both slaves. Additionally, the master could have been enabled for this shot (meaning that it would have fired again during the actual exposure rather than just communicate with the slaves via the pre-flash the instant before the shutter opened.)