206 central locking system?

i have a 1999 peugeot 206 1.1i lx. the remote central locking unit. unlocks the vehicle but when i press the button again it doesnt lock again. this is an issue as it means my boot is constantly unlocked. i have purchased a haynes manual to see if i could DIY it but it had no effect. any advice how to fix it would be appreciate it,

thanks