The best way to ensure this is to hard-close (http://support.apple.com/kb/ht5137) the Pebble app, and relaunch it . Check the top right corner of the app for the status, and tap it if it says «Disconnected Pebble» to re-establish the connection.
After this process you must request a sync from the LetsMuv app.
If you re-connect Pebble but the app still isn’t working as expected, please contact us.
It would be also very useful if you could send the incidence to Pebble ( From the Pebble app -> Contact Support )
«As per request of LetsMuv developer I’m opening this incident. It seems that DataLogging is not working properly since sometimes I need to force close Pebble App to start working again, but this will only fix the situation for a few hours/days. Thanks»