V-Pro LED dimmer problem

First ever post on here - hello everyone

I've just installed varilight V-Pro LED dimmers in two bedrooms in my flat, the LEDs installed in both rooms are the same and I've wired both dimmer switches the same way.

Problem: the dimmer in one bedroom works, but in the other room the light flickers and turning the dimmer has no effect. However, it works again when I unscrew the face plate and pull it off the wall.

Any ideas? The back box is quite full so could there be interference from any of the other wires?

First make sure you have good secure contact of all conductors in their appropriate terminals, double check no conductor cores are exposed outside of terminals/connectors and no conductor insulation is trapped within them.
I assume the LEDs are compatible with the dimmer?

Could you post some photos of the wiring, it may show a problem?

Thank you both! After PrenticeBoy's post, I checked the connections. In particular, it seemed that the neutral wires in the back box were getting squashed. I moved them to another corner where there was more space. Now the dimmer works...