drivers side door not opening

[fungus221]

some times when i go to get out of my 2002 x-type, the door catch will not open the door from the inside). this also happens on occasion when i go to open the door so as to unlock the passenger side for the wife to get in. say if i am picking her up from the shops. this means i have to stop the engine remove the key to open the door. which could be a safety problem if i needed to get out of the car quickly at any time.
any ideas as to why this should happen.
the passenger side door is always fine.