How to change a gas bottle without it leaking.

Hi all,

I've got a Topaz and I'm trying to change a lightweight gas bottle but I can smell gas afterwards and the hose screw-in joint fails the soapy water test. I keep repeating the procedure and the joint looks straight and secure but I get the same result every time.

When I screw the hose in the I push it into the joint from the metal plate behind the joint so it appears to be evenly screwed in.  Is this the correct way to screw the hose in? I've only been caravanning for a year and a bit and I'm not sure if I'm doing this right.  Would appreciate any advice from experienced caravanners.