You are asking the regulator for way too much current from a 12V supply, so as reply#1 suggested, it is overheating and shutting down. The problem is that the regulator must dissipate the heat generated by dropping 7V at the total current draw (around 2 Watts).
Power the CO2 detector and relays from a separate 5V source, and connect the grounds. That source could be a different, but more capable step down regulator powered by the same 12V supply.