H.264 Video Supported by iOS Devices
from: StackOverflow
Not all H.264 encoded Mp4 files are supported by iPhone/iPad (or Chrome for that matter) and slight differences in the encoding process can produce videos that do not work. Even if the EXACT same encoding settings were used, H.264 is a variable bit-rate encoder, so different videos may exceed bitrate limits, causing some to work and other not.
The encoding settings that were successful for us were:
•Only use the H.264 Baseline Profile Level 3.0
•Resolution below 640 x 480 and framerate up to 30 fps
•B frames are not supported in the Baseline profile.
•bitrate limit of 900kb.