At least the better ones, have a thermostat built into them to control the temperature. With the floor being held at a constant temp, it is probably not expansion/contraction. It is very possible that the heated floor has dried out the subfloor and caused it to loosen. It is also possible that the installer did not pepare the subfloor correctly, and it flexes as you walk on it because of lack of support.
The subfloor should be a minimum of 1 1/8 inches thick with 16 inch on center floor joists. On top of that, the installer should have used a 1/4 inch concrete backer board such as Hardi Backer.
If this is a new house, have the contractor come back and fix it. If you can get to the subfloor from underneath (basement) supports can be added between the joists against the subfloor to reinforce it. BUT you must be VERY careful not to use screws that will penetrate into the Flextherm and ruin it.
Good Luck!
Dodgeman
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