I was driving along the motorway. When my boot/trunk opened. I closed it again and locked it shut. But still I can lift it open while it's still locked. Why does this happen and can it be rectified.

tailgates have no lock on them unless you fit one by eagle4x4 or other companies like that
most tailgates will lock with a hard top or roller shutter but if nothing fitted then they dont lock with the central locking

Do you mean the steel tailgate door or the glass door on the canopy?