The "Honeymoon" period: As you brought your bundle of joy home, the first diaper or two worked like a charm. You thought, "Hey, this cloth diapering thing is pretty easy!" Then, only a few short diaper changes later, you started having leak issues...
In the beginning your diapers may leak until you get used to the way they work. Be sure your insert is flat inside the diaper and that you have secured the diaper tightly enough to prevent leaking. Also, some diapers and inserts do not reach full absorbency until after a few uses/washes. Have you dried your diapers in the dryer? Using the dryer to dry diapers (at least for the first time) helps tighten the PUL, which may also help with absorbency.
Use, Rinse, Wash, Repeat (again and again): Now that you are in the swing of diapering, you have gone weeks without a leak issue. But now, all of a sudden, the leak issues have returned.
After some use and washing, your diapers may need to be stripped. Soap residue from washing can build up and reduce the absorbency of your diapers. Here's one method for stripping diapers:
Wash dirty diapers like normal. Then run the diapers through a hot wash cycle with no detergent. Peek in the washer to look for bubbles. Keep running rinse cycles until there are no bubbles. (I usually run about 4 cycles.) This should remove the build-up in your diapers. Also, if no one has told you yet, diaper rash creams can create build-up in the diapers. ESPECIALLY those with zinc oxide! The best solution for rashy babies is to use creams specifically made for cloth diapers...try Mama Roses Naturals, Northern Essences Rash Salve, or Earth Mama Angel Baby creams, among many others. The kinds you can buy at Wal-Mart, like Butt Paste, or Balmex, are NOT safe to use on the diapers. The main reason for this is the gunk they leave in your diapers. Rash cream is extremely hard to get out of diapers, so I recommend using a liner when using creams. Be sure to wash liners separately from diapers if you want to keep from having any absorbency issues.
If your diapers smell like ammonia or are stained, you can put a scoop of OxyClean in when you wash diapers, and then rinse several times. DO NOT USE BLEACH! This will break down your PUL and make diapers leak like a sieve!
"I've stripped my diapers like there's no tomorrow, and STILL I have leaks!": If you have tried everything you can think of to stop leaking and are still having trouble, check your inserts.
The microfiber in your inserts can break down after repeated use, which will decrease your absorbency as well. Try testing your inserts with a cup of water. If it isn't holding enough water, you may need to buy new inserts.
Still have leaking questions? Comment below, and I (or other followers) can try to address them.