the sliding door ,although it did lock when we bought & was a bit of a fiddle, just wont lock now from outside or inside.. sitting on our drive unlocked as we speak

we've tried turning the lever a vareity of degrees, before trying to lock and also pulling the lever back. is there something obvious wrong, or do we have a knackered lock?
So in summary.. intermittent.. sometimes locks easily, but at mo not locking at all - tho inside lock nob will move around half way down but no further.
anyone have a spare door lock? nothing on ebay and will spend some time googling them.
any help at all much appreciated. we're supposed to go on holiday on thursday night
