pg_dump's table is empty

["Daniel Punton" <compass2k@xxxxxxxxx>]

I am dumping postgres 7 tables under the postgres account  mydb# pg_dump -O -a -t mytable > mytable.out and am getting either
empty dumps ( actual db tables are populated) or no dump at all.
I have file write permissions and this process has worked in the past.
Any suggestions what might be wrong?

Cheers
Dan