Why won't this app sync to my box?
What do you mean by "sync"? If you are referring to Cloud DVR recordings, the app will show up to your most recent 60 hours of recordings while the set-top box will hold over 100 hours. See the FAQ here: https://www.xfinity.com/support/cable-tv/x1-dvr-cloud-technology-general-faqs/
If there are shows that are important for you to have in the Cloud, you can change the recording option to "Keep for 1 year". This will leave less room for other recordings but gives you the option to save the content that matters most to you.
Hope this answers your question.