I think you are at the point where starting to worry is called for. An adult corn going 3 weeks without food is normally no problem at all...a baby is a different story. The GTP too some food which is great, but still doesn't mean you are in the clear.
I see the description of the corn set-up but what are the temps in there?
I would wait at least 5 or 6 day probably even a full week if it were me, but I have very limited GTP keepin time under my belt...
If the warm spot of the corn tank is big you may try reducing the size of it (pull the under tand heater out from under it a little) or kill the infared light for a while and watch the temps. Normally if a young corn won't eat it is too cold, you may be having the other problem though...possible just warmer than it wants to stay. Keep a warm area for it to use but try dropping the size of it or the ambient temps.
