Sometimes, the countdown clock may show that there's still time left to accept a ride request, but when you tap to accept, it expires. This likely is a result of a delay between your phone and our servers. This shouldn't happen often, but when it does, the ride has likely expired or been sent to another driver. Be ready to accept the next request instead, and consider moving to a better coverage area.
If you're receiving notifications that you missed requests but were never prompted to accept them, it's likely due to poor connectivity area for your carrier. Move to a different area, or try toggling Airplane Mode to reset your phone's connection to the network.