Depending on the stop policy you've configured for your populations, it can take a long time for a test to stop.
By default, the virtual users are immediately stopped. The other options are to stop the test after a configured delay or to let the virtual users finish their current iteration.
When you would like to force the stopping of a test, you can click twice or even 3 times the stop button. This will force NeoLoad to properly stop the test.
For mor information, see “Runtime supervision” under “Runtime” section in NeoLoad documentation.
