How to get air into cup?

I tried the large fleurcup for the first time today, going from the small meluna so it was quite a change. First thing I noticed was that I started feeling achy cramps after insertion, and it seemed to be tugging on my cervix. So I checked, and the cup is flattened while sealed. How do I get rid of this suction? I've tried pushing on the back wall, to no avail. It feels like it inflates, but when I stop pushing it just goes back down. I'm thinking I might be too narrow for this cup, which is a shame because I need the capacity.