The maximum measurable delay time is adjustable in the settings. Attempting to measure a delay greater than this will inevitably lead to corrupt measurements. The default setting is 1s, which should cover the vast majority of real-world situations, since it covers a distance of 330 meters.
You should ensure both source and response signals have reached stability before attempting measurement. In particular, do not stop or start the audio, change the volume or any other parameter just before or during measurement. This would invalidate the measurement and you would have to start again.
Please note there are many unknowns in play when determining the optimum delay figure. While we did our best to make this tool as robust and accurate as possible, as with all automatic procedures there is always a possibility that it will fail. In this case you should repeat the process or resort to manual adjustment until you get satisfactory results.
The major assumption behind delay compensation is that there is a main direct path from source to listener. In a very reverberant or complex-shaped acoustic space, this obviously does not apply anymore. This is where acoustics expertise and trial-and-error comes into play, in order to attain the best compromise.