Usually this happens when the computer or the phone/tablet is making itself visible to the other device. A device broadcasts a special radio signal to make it "visible" or "discoverable" to other Bluetooth devices nearby. This mode is usually turned off by default on computers and phone/tablets for security reasons.
To make the computer and phone/tablet to see each other, make sure they are in discoverable status temporarily.
Below are instructions for some common devices. For devices not listed here, see the user guide or instruction manual for that device, or search for “make <device name> discoverable in Bluetooth” on the web.
iOS (iPhone, iPad)
Your iOS device will become discoverable when you go to Settings > Bluetooth. It will remain discoverable as long as you stay on the Bluetooth menu page.
Android Versions 2.2 and 2.3 (Froyo and Gingerbread)
Open Settings. Press Wireless & Networks. Press Bluetooth settings. If Bluetooth is not already on, touch it to turn it on. Touch Discoverable. Your phone/tablet will stay discoverable for 120 seconds.
Android Versions 4.0, 4.1, 4.2, 4.3, 4.4 (Ice Cream Sandwich, Jelly Bean, KitKat)
Open Settings. Slide the Bluetooth switch to the On position. Touch Bluetooth. At the top will be the name of your device and the words “Not visible to other Bluetooth devices.” Touch this to change it to “Visible to all nearby Bluetooth devices.” Touch it again after pairing with your computer to turn it off again.
Android Versions 5.0, 5.1 (Lollipop)
Open Settings. Tap Bluetooth. You device will remain discoverable as long as this screen is displayed.
Windows 10, 8.1, 8
Press Windows Key + D to go to the Desktop. Click Bluetooth in the System Tray. Select Open Settings. Place a check next to “Allow Bluetooth devices to find this computer.” Be sure to turn this off after pairing is successful.
Click Bluetooth in the System Tray. Select Open Settings. Place a check next to “Allow Bluetooth devices to find this computer.” Be sure to turn this off after pairing is successful.
Right-click Bluetooth in the System Tray. Select Bluetooth Configuration. Click the Accessibility tab. Place a check next to “Let other Bluetooth devices discover this computer.” Be sure to turn this off after pairing is successful.