Follow these steps,directly from PEBBLE's site:
http://help.getpebble.com/customer/portal/articles/1727420-android---troubleshooting-
Pebble will not pair / connect
If you have already paired Pebble with one or more devices, your attempts to connect on your Android may be blocked by a previous pairing trying to re-connect or if a severed connection did not fully register on one of the devices. Please unpair / forget the pairing from each device.
To unpair devices, follow this path on your Pebble: Main menu >> Settings >> Bluetooth >> [Device’s name] >> Forget.
To unpair Pebbles on your Android devices, go to Settings >> Wireless & Networks >> Bluetooth >> hold on [Pebble’s identifier] >> Unpair.
To make sure that your previous connection has been fully severed, please restart each device. To reboot Pebble, please press and hold the Back and Select (simultaneously) for 10-seconds.
Once you have successfully unpaired and restarted each involved device, try your pairing again.
Launch your Pebble app
and then
at the bottom of your screen to start the pairing walk-through.
Make sure that you Pebble is turned on and nearby.
Your Pebble app will automatically look for and list available Pebbles. When you see
your Pebble's identifier listed, tap it to pair.
Confirm on your Android and on the Pebble itself that you would like these two devices paired.