UFH electrical connections problem

[Tipper]

Hi, I'm having to replace my UFH heating controller due to a fractured connector block broken by the hamfisted installer. I thought it would be a quick swap job but the replacement unit has this connector block fitted for various connections but I can't get the cables to hold. I've got loads of wires to fit as it's under floor heating - all those zone valves!
I've tinned the flexes with solder to stiffen them up but as soon as I release pressure on the little square button the cable can be pulled out.
What am I doing wrong? Is there a trick to these connectors or are they just crap?
It's a Wavin/Jablotron AC-116.
Thanks

 

[Tipper]

Sorted.

Just needed to push really hard! I didn't initially because I didn't want to break either the connector block or the pcb.