remote central locking fault

can anyone help me i have got a 1990 range rover vouge se 3.9 efi i have just locked it with my alarm fob and i cant same to open it again with the same fob the fob is rectanglar in shape with land rover in the middle printed on in green and a little red light above that, dont know if it has lost its code because the red light still flashes when the button is pressed if has lost its code how do i recode it

all help is apperciated

are the locks stiff, i had a problems with mine that my passenger side wouldnt close with the central locking, it might just be down to the fact that the locks need oiling.
do all the locks open when the key is used in the drivers side???