Do you have your device set to "use network time" or "automatically set date and time" or whatever that is?
Your phone time needs to be absolutely sync'ed with server time, because that's how it knows when to start and end the battle and selection. You basically want it to be within a couple of seconds of the actual time. It got turned off for me, and the next day raiding, I was entering the battle a few seconds after everyone else. If you were off by a few minutes, you wouldn't be able to raid at all.
Note that on a device with mobile internet, this setting will "unset" itself if you go into areas where the only coverage doesn't support it (at least on my Galaxy S7 active)
