Hello My name is Ian ~ I am not on the site constantly so please be patient whilst waiting for responses to you queries
OK can you just confirm that you are using it on a 21mm gas cylinder fitment as many companies are now using the 20mm sized one, whos the gas supplier?
The gas supplier is Color - the 15kg clinder
The liner is 21 and the reg. is type 634 and seems to fit perfectlycylinder is 21
OK you should be able to depress release lever and twist the locking knob around which then locks on, if you cant do this then you need to replace the coupling