It could be the centre pin on the carto has been pushed up a little and needs to be pulled down again. The lavatube has a pretty deep connection cup which creates this problem sometimes.
If you look at the bottom of the carto, you'll see two "flaps" of metal sticking out slightly. Pry them down slightly with a thin screwdriver or something similar. If you pull them down too far it could pull the pin out completely. If you pull them down a little it should be able to make contact with the lavatube's connection too.
I have to do this for some of my CE2s but it's easy enough if that's what the problem is.