Been successfully roasting green beans at home for a few years now. Just discovered my first problem - no first crack!

Following a tried and true recipe on an SR540 my beans went from green to straw to various shades of brown but the usual popcorn sounds of first crack are notably absent. Just a few beans do it. Like, 5 pops instead of hundreds. 2nd crack happens as usual.

I'm still early into my 10 lbs bag, and it didn't happen with the first few roasts of the new bag.

Any ideas? Premature failure of the SR540? Bad beans? Stroke? Anyone else ever have this happen?