A yellow blinking light indicates the vent can't find the bridge. This typically occurs if the vent is on the cusp of the bridge range and experiences some 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.
There are a few ways you can go about fixing this. There's a blind spot in back of the bridge where the cords plug in, so it's important to make sure the opposite side (the 'front') is facing as many devices as possible.
The range of the bridge is typically about 30-60ft, so you should also try to ensure it's as close as possible while still connected to an ethernet port and power outlet. This video can help walk you through best practices to get the most out of your Smart Bridge range:
If you have the bridge pointing at the vent and it's as close as possible, you may need a second bridge to act as a repeater. A repeater would only plug into a power source (no ethernet), and then it takes your main bridge's existing signal and extends it outward.