How can I make my Mazda 2 boot stay shut?

I have a 2007 Mazda 2, 5-door (newer model). When first setting off, the central locking sometimes clicks & the dashboard warning light comes on. On inspection, the boot door has come undone, although is still held closed by the safety catch. None of the other doors are affected, and it happens even when I haven't opened the boot. Any ideas on why & what I can do to fix this please?