My APP check update by comparing local version and remote version returned by iTunes lookup API. But the API still return old version after new version has released.
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I was facing the same issue. It has two steps to resolve this issue.
I was using http://itunes.apple.com/lookup?bundleId="YOUR BUNDLE ID" not the https.
Step #1: Please ensure that you are using https not the http because it will not give you the updated version number. First i uploaded my app with version 1.8.0 then version 1.9.0 then version 1.9.1. When i used http it was always showing me 1.8.0 even the app on app store had the version 1.9.1. So, just use https for future.
Step #2: After uploading the latest version please wait for 24 hours in order to get the latest version on https://itunes.apple.com/lookup?bundleId="YOUR BUNDLE ID". I was getting version 1.9.0 even though the app was live with version 1.9.1 within 24 hours. I waited for 24 hours and then it started giving me the correct version and then i was able to force update my app by version checking.