If you hear the fan speed increase slightly somewhere between first and second crack, the Popper might have hit it’s temperature limit. Here’s a video showing what it sounds like, and what to do…
The Popper coffee roaster has a thermal switch which prevents the roaster from getting too hot too fast. This can kick in some time after first crack, as the machine nears second crack. It’s annoying perhaps but required to have for UL certification.
If this is causing issues, you might get better results with a slower heat ramp getting to first crack, and slowing down the roast between 1st and 2nd cracks. We have a page/video all about that: https://library.sweetmarias.com/roast-development-to-second-crack-in-popper-coffee-roaster/
If it’s still an issue it is quite easy to modify the position of the switch a bit, pushing it up higher on the roast chamber. But opening up a roaster means voiding the warranty so consider that! Here is a video dedicated to explaining the thermal cut off switch and how to address the issue.