You are setting all recordings from the guide, not from an ipad or similar? I believe the automatic thumb only happens for 'local' recordings.
Another possibility is it doesn't bother adding a thumb where it would have no effect. IIRC, single thumb-ups on non-series recordings are ignored by the Suggestions logic.
Obviously you are using the thumb for a purpose it wsn't intended for. There is a very easy workaround, though.
