The Recordset is Not Updateable

Help! For some reason one of my forms has stopped working in that I can't
change any values in the fields The status bar says "The recordset is not
updateable". The form is based on a qry and if I run the qry itself I get the
same message when I try to change values. This has worked fine up until now
and only stopped working a couple of days ago. I haven't changed the table
design at all. The qry is based on three tables all linked by the REFNO
field. tblCompany has a primary key of REFNO and EMS and REF have a foreign
key of REFNO. If I remove either EMS or REFNO from the qry i can change data
but having both causes the error. EMS and REF are both linked dBaseV tables
also being used elsewhere. I don't think this is a locking issue because it's
been happening for a day or two now.

Can anyone help at all? If it helps I can paste the qry here?

Thanks
Paul