First, go in the Philips Hue app to Settings > Light setup > tap on "+" and search for your lights automatically. When the light can’t be found you can select the manual search option and follow the onscreen instructions, for this you need the serial number of the bulb. The serial number can be found on the lamp base, which is a 6 digit code.
When you are experiencing issues with unreachable lights in your App, we recommend to check if they have electricity or they may be turned off with the wall switch accidentally. The lights have a maximum range of 10 meters to the bridge from the first light point, when you have more lights it is important that you extend the range with lights in between for example to ensure the signal can go to every light point in your house (this called ZigBee mesh network). You can always try to do a ZigBee Channel change in case above is not working for you via: