Why does an automatic bluetooth connect fails after some time?
This can be due to many things, without more information it is impossible to pin-point where the problem is coming from.

Some common problems might be:  


Charge up the device you're trying to pair. Some devices have smart power management that may turn off Bluetooth if the battery level is too low. This may cause the connection to terminate unexpectedly.
Clear the Bluetooth cache (Android only). Sometimes apps will interfere with Bluetooth operation and clearing the cache can solve the problem. Go Settings > Backup and restart > Reset network settings. 
Sometimes pairing of another device with your car's Bluetooth may lead to your original device being unpaired. Make sure that no one else is pairing their device with your car.


Source: Here