PD-6WCL dimmer stops working when branch circuit connected

In a 3-gang switch box, I'm wiring three PD-6WCL dimmers that are controlling one light each. I have identified the hot (line) wire which is also connected to another cable that seems to power another independent switch box and lights in an adjacent room. There were three standard single-pole switches in this box previously working fine.

When I connect the three dimmers to the hot (line) everything works as expected. When I add the branch line one of the dimmers stops working. I see one LED on the dimming indicator and it doesn't power up or switch the light. It's always the same light that doesn't work regardless of swapping switches. If I remove the branch wire everything works.

Any advice or idea what might be going on in this situation? Thanks!

It sounds like you did everything correctly. First, I would check the neutral wires. You didn't touch them but sometimes just pulling stuff out of the box and putting it back in can cause something to come loose. Second, I would twist the hot and feed to the next box then add the dimmers one at a time.

Looks like the cause was bad or marginal wire connections. It's odd that it was so consistently affected the same switch circuit. But, I replaced all wire nut connections with WAGO 221 connectors and everything works. I know this is normally done with wire nuts, but with the amount of pig-tails and wires involved I must have not been able to have a solid connection somewhere causing the dimmer to fail.

I actually like the stab-in connectors...IF... you use the proper size!