After following the great tutorials from http://www.raywenderlich.com/ on how to submit your app to the app store from square one I started planning my beta test program.
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We have also faced with this problem couple of times and possible reasons might be;
Due to the screen shots, did you upload your files to exact location "http://www.becomingapps.com/en/ota/"
location? When you give this URL during enterprise export.
Second possible reason we have seen is mismatched App Id's
.