By "pop" do you mean hear it go down? Or do you mean it doesn't click when you press the top the next day whether or not you heard any popping down?
Nearly always, I can get the tops off with my fingers. Technique matters a lot, I think. And finger strength. Some are easier than others. As long as a top doesn't click down when I press its middle before I start taking it off, it was properly sealed.
Well, it was still safely sealed anyway. "Properly sealed" could arguably be different. If it is too lightly sealed, it might unseal when the jars are moved or temperatures where it is stored change.
Lightly sealed could be from variations in the lid. Or variations in things like the amount of headspace, temperature of the jars or contents when it goes into the canner. All those vary for me because I use different brands of lids, reuse lids sometimes, and sometimes I don't get a batch from warming the jar to filling the jar or from filling the jar to putting the lids on as fast as I do at other times. I am usually pretty precise about how hot the contents are and how much headspace there is.