Your access token might have expired. You need to fetch a new access token. An access token is valid for one whole day. Once you get an access token, you don't have to request it again unless you are logged out.
A request token is valid for a couple of minutes and can be used only once. You should not request for access token for every run. Make sure you store the access token in preferences or a database and reuse it in the next run, and don't call generateSession for every run.
A request token is valid for a couple of minutes and can be used only once. You should not request for access token for every run. Make sure you store the access token in preferences or a database and reuse it in the next run, and don't call generateSession for every run.