With push fit to release the cable you normally have to press a small tab, make sure that it isn't pressed down.
If you are trying to connect flex into it then that can sometimes be difficult, twist the end nice and tight before you try and get it into the push connector.
Failing that remove the pushfit connectors and wire in some standard 6a connector blocks.
If so they're absolute garbage. You need to use a single 1.0mm² solid core conductor into each terminal.
Strip about 10mm of insulation, and shove it in the terminal. Keep trying until it grips, as there seems to be plenty of room in the terminal which does not grip the wire.
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