This is a bug that has been reported in version 1.00.39 and earlier.
We've gotten reports of some levels where patrons don't show up but the timer just ticks down or the UI says there is one more patron that will come into shop.
We've been able to identify what is causing this bug and will be releasing a fix for this soon. Please bear with us as we improve the game!
If you encounter this bug, please send us a screenshot or recording along with what level you're on via email (support@monstronauts.com) or on our official Discord community.