Can anyone advise me on a GAS problem I have??

[viperbl]

As you can see I have a device that allows two gas bottles to be attached (one feeding at a time using that little black knob to select.

I replaced both canisters at the weekend and then no flow at all ...

Now, I did a little test, I removed the valve device from the wall from the feed to the hob, I have attached just one canister and turned it on, it has taken the pressure and the yellow tap is turned to on, but there is no gas coming out of the bottom ... should I expect gas to come out of here or does it require a certain amount of back pressure to allow it open?

Under the yellow shut of valve you can see what looks like a bleed screw, unfortunately before I could turn this it came off in my hands ...

My thinking is I may need to use this to bleed the pressure into the system before it will work ... with this broken, I assume its a new system ????