How old are the covers? All covers will eventually wear out and ME only guarantees them for something like 6 months of us. BUT I have covers that have had about 3-4 years of constant use and are still going strong and that's not at all unusual, most people find they can get similar wear time out of them. The print is faded and the inside is quite yellowed but they still work as well as ever. So unless yours are quite old or weren't treated very well (ie frequent hot water washes, bleached frequently and/or frequently dried in a hot dryer) then I wouldn't expect that they'd be worn out yet. Not impossible though, really depends...
I wouldn't necessarily suspect build-up in the diapers either, not unless we're talking a heavy fabric softener build-up. Detergent build-up can cause leaking in pocket diapers because the fleece is prone to repelling with the least bit of built-up. But it would be unusual for a fitted diaper to repel that way just because of build-up, you'd get things like nasty ammonia odors and rashes instead. All that said, if you suspect build-up then a good stripping certainly can't hurt. Do you use a fleece liner in your diapers though? If they were to get build-up then maybe they'd be repelling and not allowing urine to soak through to the diaper quick enough... Don't know how likely that is but it's a thought.
Is this happening with all your covers or just one in particular? When you say soaked, just how much are we talking here and where? Are the clothing completly soaked everywhere? If it's not totally soaked, are the wet spots along where the edges of the cover would be (like around the legs or the waist)? Or is it somewhere else, more in the middle of the diaper area? Around the edges of the cover would seem to indicate that the cover is working alright but there's just too much 'free-floating' liquid inside that it can't hold in so it's coming out the leg holes and possible the waist (if say she's lying down). Wet spots in the middle of the cover would indicate that liquid is actually soaking through the cover and onto clothing. That could indicate the cover is worn out or perhaps has a small pin-hole somewhere.
I know it can be hard to tell what exactly is happening, especially if the clothing is really wet everywhere by the time you see it. One trick I learned is to take a clean/dry cover and a clean/dry diaper (or towel, anything thick and absorbant that can be put into the cover will do). Wet the diaper/towel and then wring it out, you want it pretty soaked but not dripping. Then place it inside the cover and then close it up. Gently apply pressure here and there all over the cover and see if any wetness comes through. If it's a pinhole or too much wear, then you should see moisture wicking through the fabric at one or several spots. If it's some kind of lack of absorbancy in the diaper then nothing should happen (because although there's plenty of liquid, it's all locked in the diaper).
What diaper are you using? When you get these leaks, is the diaper totally saturated or are there dry spots (like around the waist and at the back)? Sometimes toddlers do need to have a liner placed in every diaper because they just pee too much at one time for any one diaper (usually this would happen with the One-Size as the Lg Sandy's is more absorbant).
Hope that helps somewhat,