A yellow blinking light indicates the vent can't find the bridge. This is either due to range or interference. Since the vents sit in a metal duct, there are certain angles that significantly decrease range. A diminishing battery life can also reduce a vent's reach if it sits on the outer edge of the bridge's signal range.
At most you’ll need an additional bridge to act as a signal repeater, but we’ve been able to solve issues like this without requiring additional hardware. This video can help walk you through best practices to get the most out of your Smart Bridge:
If you're still experiencing trouble, please contact support for further assistance.