There are several reasons this can happen, here are a few ones:

1. Bundle ID issues -

        Generally, this issue occurs when the Bundle ID is incorrect.

        Make sure the Bundle ID is correctly defined using the following checks:

  • The Bundle identifiers in the .ipa file are written out in reverse DNS notation (i.e com.myCompany.myApp)
  • The Bundle ID string format should be Uniform Type Identifier(UTI), which is alphanumeric characters (A-Z,a-z,0-9), hyphen (-), and period (.)

2. Filename issues -

        The .ipa file name must not contain any spaces/special characters

3. Network unstable - 

        Find a stable network connection which provides a minimum speed of at least 3 mbps

Also, please try the .ipa file locally on a real iOS device (not simulators) before trying it on any pCloudy devices.