Hi Mah Yon Fong,
When you say the app can't access the RSS data, do you have errors in your log? Do you see anything in your console? Do you have the particular code that returns an error, can you provide anything about what causes a problem or what you are doing in your app?
Honestly, as a question, this is unfortunately impossible to answer with more information. It is possible, like barryhunter pointed out, that you're being hit by the deprecation of the Master/Slave datastore, but it's hard to tell right now.
If you could include the errors in your logs (
console.developers.google.com, choose your project, then "monitor" then "logs"), and the app.yaml, with the relevant handler code, it would be helfpul to troubleshoot further
Cheers