JD, I believe this is a result of the tape placement more than the diaper's basic shape. Of course this often translates into the diaper's overall size too. Let me explain.
First and foremost, a diaper needs to fit like a diaper in order for it to work right. A lot of people don't even think about this and try to go for a diaper that fits more like brief underwear.
A diaper "should" extend all the way up to a persons waist (which is around the belly button), which is what every diaper manufacturer bases their sizing info on. However, most people are going to be used to underwear that only extend to just above the hips line (more properly called the high hip line- but certainly not the waist line). When they try to do this with a diaper they are also (unintentionally) getting a diaper that is one size too small. However, this not only lowers the diaper's rise, but also shrinks every other measurement. This in turns makes a diaper tape on more tightly all the way around the sides and buttox, which is what's actually popping those tapes off prematurely- especially the lower tapes (it's also what causes a diaper to split open along the back side should the tapes manage to hold). The way a few manufacturers have compensated for this, at least a little, is to raise the positioning of the lower tapes towards the middle of the side panels- and to include a stronger tape landing zone. This makes it so both the middle and top tapes are now more likely to not both pop off. Which is where most of these people end up at (especially if the diaper you're using doesn't imply this compensation), having to reinforce those tapes because they still keep popping off all of the time.
To really prevent this from happening, the diaper "should" be properly fitted and snug all the way around the sides, at the waist, and all the way around the legs too. None of it should feel tight or too loose, and the diaper should have at least some room to move, expand as they should, and/or easily contain large bowel movements without leaking.
With this proper fit the tapes on most any non-compensated diaper I've ever tried will not have a problem popping off. Though yes it does still happen from time to time. Now with a diaper that is compensated for a smaller fit/ no lower tapes, when fitted properly they will never allow for a snug fit around the legs, but wow those tapes will hold. (Edit: Adding a piece of tape for a third, lower tape to these diapers does un/re-compensate for that improper and loose fit around the legs and makes it fit snug while also preventing premature leaks. This alone has worked great for me to get these kinds of diapers to work properly).
It's also very much worth mentioning that every diaper is cut differently and every person is shaped differently too. Some diapers out there will just never fit some people no matter what. Also, like the diaper primer mentions you need to always be careful not to get oil from you hands on the tapes or on the plastic where the tapes will go (such as rubbing the panels flat just before you tape it). I've also found it helps greatly to rub the tabs after you've stuck them on. This makes sure they are fully contacting the diaper, and might also partially melt the sticky glue into the plastic which would improve its holding ability as well.
Of course this is all just my own observations, but it does seem to explain a lot and has seemed to be very consistent based on what I've been able to verify. But that's just me and I could be wrong.
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