lvextend problem in linux 2.6.7

hi,
i created a volume group(vg1) and a logical volume(lvm1) for two
partitions. i am able to add and remove one more partition to existing volume group (vg1). But i am unable to extend the logical volume (lvm1) and getting errors while using command lvextend.
i am using linux 2.6.7 kernel and installed device-mapper and LVM2 from http://sources.redhat.com/pub/lvm2 and http://sources.redhat.com/pub/db

please reply me asap,

regards
sridhar.