The Flexpass doesn't beep in some Express Lanes. It works fine but doesn't beep. So don't use beeps to determine if it's working or not!
Here's text from the Fastrak Handbook page 6: "Will the toll tag beep in express lanes?
Depending on the express lane location, the toll tag may or may not beep. You will only be charged for your trip’s toll amount regardless of how often your toll tag beeps."
I was trying all sorts of positions to get the tag to work on the new 680 express lane in our 2013 Model S. The assumption was that if it worked, it beeps. Then I noticed it does't beep in our other brand EV. I called Fastrak and the customer service expert told me that if it doesn't beep, it's not working. I mentioned it beeps at the bridges, the Sunol grade express lane, etc. Just not on the new 680 express lane. He said its probably a bad tag.
Looking at the transaction report there were no charges for when the tag was mounted to the black area on the passenger side of the mirror, turned 90 degrees as motioned above. There were also no charges when using it in the other car. So it's working. I did a web search and found the above text here
https://www.bayareafastrak.org/en/images/handbook.pdf
Apparently the nice and helpful Fastrak customer services person is not aware of what's in the Fastrak handbook.
So the measure of "working" is not whether it beeps or not! It's how it charges. Try a recommended position and logon to view transactions to see if it charged or not.
Another tidbit found on the Fastrak site:
https://www.bayareafastrak.org/en/images/MetalOxideWindshieldList.pdf
Here's what the list says about Tesla vehicles "Exterior Toll Tag Required - Windshield prevents use of interior toll tag".
An exterior tag will not work in a Flexpass situation.......
Also note the numerous other fine vehicles with issues.
It appears there are some systemic engineering issues for in front of them.