Difficulty riding in ERG mode

Hi, new Zwifter. I have an Elite Directo XR. I have calibrated the trainer on Elite’s App. I have calibrated it on Zwift. It’s connected to Zwift by Bluetooth and the Controllable is paired. Today, just now in fact, I tried it in ERG mode for the first time. The difficulty I had is that the Controllable was over compensating to adjust power output. Typically a programme starts with low power and high cadence during warm up phase. I found when I started the power output was too high, the brake would then be applied which reduced my cadence. The result was very slow cadence, sometimes 20rpm or less to maintain the power. It felt like I was pushing a big gear. The only way to stop this was to stop pedalling, and start again. I tried starting slowly but as the power increased the brake was applied often before the screen showed the power output reaching the desired output. I made several attempts. All failed. I was on the small chainring and a big cog. What am I doing wrong?