There's a few things that will make them stop eating

If temps are too low or high then this will cause stress and decrease appetite but some gannets may still eat even if they are over or underheated. Another is to remember that when older they need less food and might not always take it unless you give them a few days without. Like I said MBD is one thing that can make frogs lethargic or have difficulties with feeding. Basically, read up and go over your husbandry make sure every times right and check for signs of metabolic bone disease