1) Cloth Diapers might leak if you do not have a good fit.
Cloth diapers need to be quite snug around the legs and waist. If you can easily slide a finger under the fabric it is too loose. You don’t want to cut off the baby’s circulation but you do want to seal in all that poo. If your diapers are too loose, try to use a tighter snap or velcro, or you might need to try a different brand that fits better for your baby. Make sure there are no gaps, especially around the legs. On more than one occasion I have put a diaper on too loose (especially with velcro) in the middle of the night when it was dark and my brain was all foggy. This unfortunately led to the horror of waking up to a poop covered child and bed. Blegh!
2) Cheaply Made Cloth Diapers can leak.
Sometimes (not always) cheaply made diapers don’t fit well, and have cheaper materials. In the Cloth Diaper world you often get what you pay for and investing in good quality, well-made diapers is well worth it. They will last much longer and will hold in messes! I’ve definitely noticed a difference between my cheaper diapers compared with higher quality brand names or hand made WAHM diapers.
3) Old, worn out, diapers can leak
I have purchased a few second hand diapers at a nice discount. For the most part they have been great but on some, the elastic has worn out and does not sit snugly around baby’s waist and legs. This has also led to poopy blowouts. The elastics can be replaced if you have some basic sewing skills. There are lots of youtube videos (like this one) that can sow you how to do this. Or check out this tutorial with pictures.