first enter these commands into console:
cl_allowupload 1
cl_allowdownload 1
cl_download_ingame 1
wait a couple of mins to make sure your spray is fully uploaded and then try spraying.
if that dont work it might just be that the server you're on has sprays disabled so start your own server, type the commands in and try again.
if that dont work then the spray you're using is probably wrong file size, image size, bit depth, or something like that.
try using wally if your not using it already cos its quick, easy and checks that the spray size, filesize and bit depth before you save.
www.telefragged.com/wally