Check if your data connection really is good, and terminate the app and re-boot it in two cases:
1. Kill the app and re-launch it before leaving the house or your place of work/school, after it has been running in the background without you moving for many hours. Get a fresh re-start before you resume playing the game
2. Stop it and relaunch it if anything goes buggy in any way, or lags too much. When i get no pokemon, or a pokestop goes buggy on me, or when a fellow player who is 1 meter from me gets a spawn and i do not get it (up to 20 seconds delay can happen, above that you need a relaunch), i go to the applications manager (android), force a shutdown, and start it back up.
You are doing it right when you see the launch screens with the progress bar and Niantic + Niantic logo + The Pokemon Company on that white background.
Try it, when in doubt kill the app and restart it. After trying this, do the problems persist?