While Seriously Simple Podcasting makes every effort to ensure that your podcast feed is valid and acceptable to iTunes, Stitcher, Google Play and any number of other podcasting services, there’s always the chance that something can prevent things from working as expected. If you receive an error from iTunes saying that they cannot accept your feed, or they cannot read it for a supplied reason then the first step is to run your feed through a validator to check that it is formatted correctly.
If you are using iTunes, then iTunes Connect (the iTunes podcast manager) will provide a validation service for you. Similarly, Google Play Podcasts will also handle the validation without you having to look elsewhere.
If you are not using either of those services, then the most reliable and up-to-date validator for podcast feeds is the Cast Feed Validator, so go there and check to see the results of your feed validation. Once you have done that then you can log a support ticket and supply the link to the validation results (just copy it from your browser’s address bar).