samba - Jan 2007 - Write list options Dont work

Hi

i have a shre like this:

[DATOS]
path = /data/datos
comment = QAQC y SHA
locking = no
admin users = zulloa2
write list = @qaqc, zulloa2
browseable = yes
public = yes
guest ok = yes
force create mode = 775
force directory mode = 775
read only = yes
force group = qaqc

but some time, some files appears in read only for the user zulloa2, this user
is in the write list as you can see abve.

what can i do

thanks