Corsa passenger door locks sometimes?

I have a problem with my Vauxhall Corsa 2002 plate SXi (Corsa C) in that sometimes when I lock the doors all of the doors lock except for the passenger door (the locking sound is slower too if that makes sense). However, if I unlock it and lock it again, all is fine.

I admit this is not too much of a problem, but what can I do to get rid of this annoyance? because one day I may forget to repeat the procedure leaving my vehicle a security risk.

Thanks ;)