It could have something to do with your "stuffers". When using a "stuffer" with a diaper to increase capacity you can leave yourself susceptible to leaks due to the extra material defeating the standing leak guards. You are adding thicker material to the interior of the diaper which then reduces the overall effective height of the leak guards. If you then couple the reduced effective height of the leak guards with high volume intense voiding ("flooding"), urine can easily flow straight out of the diaper. Most "stuffers" are made with a fair amount of SAP and don't exactly have the quickest absorption rates.
Even without flooding, you can have big holes in your padding system that will allow for leaks. As your booster begins to absorb and swell, it is continuing to reduce your leak guards height more and more. You can even reach a point with the leak guards have lost all contact with the skin, and possibly even the leg gathers as well.
Using these capacity boosters can give you overall gains in absorbency but can also sacrifice security and absorption rate. The best ways to increase capacity are ones that don't interfere with the leak guards. For example: It was very easy to separate the inner sheet from the outer plastic shell on the plastic backed Depend diapers. Pulling it apart in the back with your fingers would expose the absorbent material from the waistband. You could then insert your desired booster (I used Tru-Fit pads, Depend Boosters, and Abena). This would dramatically increase the capacity of the diaper while not changing the interior dimensions in the slightest. The pad was underneath the top layer.
I have also had good success when starting with a diaper that has very tall leak guards. Molicare Superplus used to have some exceptionally tall leak guards coupled with a very wide crotch.