Some tips to troubleshoot Bluetooth issues include making sure the Bluetooth is on, making sure the device is in discoverable mode and powering the device off then on. It is also possible to delete the Bluetooth device from the phone and then rediscover it.
The first thing to do is to check is that the Bluetooth is on. To do this, look for the Bluetooth symbol at the top of the phone screen. If the symbol is not there, change the phone settings. If changing the settings doesn't work, put the Bluetooth into discoverable mode, and then try to find the device again.
Powering the Bluetooth off and then turning it back on may also help troubleshoot a device. Do this by putting the phone in airplane mode and then back to regular mode. If this doesn't work, try deleting the Bluetooth device from the phone. On an iOS device, do this by tapping the name of the device and then choosing the option to unpair it. It's also possible to fix Bluetooth issues by downloading new drivers, updating the phone's hardware and ensuring the devices are close enough in proximity to communicate. Typically the paired devices should be within 30 feet of each other.