Follow along with the video below to see how to install our site as a web app on your home screen.
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I don't know what's wrong, but before panicking I would suggest uninstalling and reinstalling FPSE. If it still doesn't work, consider emailing the developer with details about what you're trying and what messages FPSE is giving you--it's possible that compatibility with your phone was broken in one of the updates but the dev won't know unless someone reports it.