You are not doing a COPY ONLY backup when you take the full back up are you? If you are, that would explain the problem. You can find if your last full backup was copy only using this:
SELECT TOP ( 1 )
FROM msdb.dbo.backupmediafamily bm
INNER JOIN msdb.dbo.backupset bs ON bm.media_set_id = bs.media_set_id
WHERE database_name = 'YourDatabaseName'
AND bs.type = 'D'
ORDER BY backup_start_date DESCIf it was not copy only, then I don't know what else might be causing this problem. Look at all the columns (SELECT *) in the query above and see if any of the fields give you any clue as to what might be wrong.