Earlier this week, I was having the issue where Zwift would detect my Wahoo Tickr, but it would never display HR - always said 0. I tried restarting, changing the battery, but nothing was working. Then yesterday, I happened to open the Wahoo App to see if it was working there and was prompted to do a Firmware update from 1.11 to 1.12. After applying the update, I opened Zwift and the HR started working immediately.
So, if you are having issues with your Tickr, check for a Firmware update and hopefully that works for you!