Unfortunately, the resistive screen technology does suffer from "calibration drift" over time. (This is the price we pay for the excceptional readability in bright sunlight--capacitive screens are still not as good, and were far, far behind in readability when the 7x0 units were originally released.)
The re-calibration method mentioned above can compensate for the drift if it doesn't get too bad, but over time the screen can continue to degrade and may eventually reach a point where the re-calibration technique won't work any more. If that ever happens, call the iFly folks and leet them know what's going on. I think there are a couple of other last-ditch things you can try, but they have a low probability of success, and you may eventually need to consider sending the unit back for a screen replacement, or trashing it in favor of a newer unit.
You may also choose to use an iPad, Android tablet, or Windows device to run iFly instead. Just make sure the screen is bright enough to read in a sunny cockpit, or figure out how to shade it.