Are you using the base 5 digits or trying to enter the Zip + 4 (provided you are in the US). Some apps aren't set up for the extra digits.

Another odd thing I've seen is where you would need to use the "master" zip code of sorts. Like my town actually has multiple codes depending on what section of town you live in (we used to have a handful of small rural post offices before they consolidated down into one big one). 29506 is still accepted in the postal system, but may not be recognized by third parties so you sometimes have to use the 29501 code that was originally assigned to the main post office.