Do your fields accept input using the test methods in interrogation mode? I have seen some screens define the fields incorrectly, so they cannot actually accept input. If that is the case, you might interrogate them as regions.
Does it work when using the test methods in interrogation? Try entering the credentials manually and before you proceed to the next screen, define a region for the area where the credentials appear. Try using the test methods to update those. Are the bounds of the region the same as the bounds of the fields you have defined?