My 11 week old daughter has silent reflux and is on 400mg of Losec per day. She has been on Losec since 2 weeks old.
You say the dose is 400mg? I think this isn't correct, as it would be 20 times an adult dose, can you recheck what dose is she on? A lot of the time if the losec is working but just not all the time, it is the dose of medicine and it may be time for an increase.

Otherwise how do you give it to her? Is it a tablet dissolved or a liquid?