I have to admit I can't quite figure out SC right now. Either I'm having a hard time hearing it over all the noise or my popper isn't getting hot enough. I'm not sure right now. More practice is called for!
There are several options:
1. FC and SC are running together (it happens).
2. You are ending the roast too early to get into SC.
3. SC is happening and you just aren't hearing it.
3 is the most likely, followed by 2, followed by 1. Here are some suggestions you might consider.
First, to address 2, intentionally take some beans 60 or 90 seconds longer than any other roast so far. You will ruin the beans, and the longer you go the greater the risk of fire (the beans will ignite at some point, and I don't know about your popper, so keep a fire extinguisher handy).
To address 3, I have a couple of ideas. SC is definitely more subtle than FC. A lot of roasters say the way to tell is that FC sounds like breaking pencils while SC seconds like breaking toothpicks. That is apt, but, for me, a better way to distinguish is the frequency of the cracks. Imagine a scale of "crack frequency" of 1 to 100. If FC is 50, SC, is 75.
Run a batch with the ambient noise reduced as much as possible (if you are roasting inside under a vent hood, consider moving outside for a trial run). Run the batch up to FC, and then do two things: close your eyes, and focus not on hearing cracks, but on
not hearing cracks, i.e., the absence of cracks. You want to hear when FC ends. Do this by counting in your head, when FC is very active, from 1 to 5. Pick any starting point and just start counting, 1...2....3. Every time you hear a crack, start over. So, 1...2...[crack]...1....2...3.....[crack]....1. Etc. When you get to 5 (or, maybe even 3 or 4 given your quicker roasting times), FC is over and the very next noise you hear will be SC. With the counting clue to help you, I think you will be able to tell that SC is very different (lighter but quicker) than FC.
After a while, you will start to judge SC by how the beans look, not sound, but you have to start with sound. And, it's important: SC is where the magic happens, even for someone like me who typically likes lighter roasts.
Another clue is simple time. If you are starting FC at 3:45 or 4:00, my guess is that by 5:00 or 5:15 you are definitely into SC.
And, just to confuse things even more, sometimes there is a third crack, which is just another SC. But don't worry about that yet.
As with everything to do with coffee, trial and error with recorded data will teach everything. Good luck.