Rob, the first thing which you need to test is if your connections between Zwift and the bike are set correctly. When you open Zwift, you see the connection screen. Your Kickr Bike should appear on three tiles, top left and two on the right-hand side in the second row: power, cadence, and controllable. The name of the connected device should be the same in all three tiles. (KICKR Bike can connect via ANT+ or Bluetooth, and the name under which it appears depends on the chosen connection). If something on your connection screen looks differently, reconnect. The controllable tile is the most important, if your trainer is not connected as Controllable, Zwift will read power but will not be able to control resistance.
The second thing to consider is to make sure that your ERG mode is turned on. On the screen where you pick a workout there is a radio button at the bottom of the window next to the word ERG. I would guess, ramp test should automatically turn on ERG, but who knows. If you do not turn on ERG, you would have to adjust resistance by changing gears on your trainer, but this is not how you take the ramp test.
The last thing, when you start the ramp test, keep your cadence at an average comfortable level, say between 80 to 90. As power increases along the ramp, go to a higher cadence, probably to a 100 or so. Do not start spinning like crazy at low resistance during warm-up or early in the ramp, this may confuse the algorithm of the trainer or Zwift, and ERG mode can get disabled. If you feel that you start spinning out, drop the cadence to like 50 or so, and wait until your trainer and Zwift reingage ERG mode.