Rather than blindly ripping door card off, I tried what both you and "Rupert" could have done - a quick internet search.
Pretty quickly I found it is a common problem, with lots of sites and forums mentioning it.
I THINK you may have the kiddie lock turned on, on that door.
Read the first few posts here:
And a few posts here
More, from YouTube:
Loads more out there for those prepared to look