Try drying your FB on HOT for 10 min or so, this will help seal any holes.
Also what detergent are you using??
If it includes ANY softeners at all it will make diapers not absorbent enough. And any "natural" detergents except for Allens or Sensiclean will make the fleece on the FB roll off, instead of, absorb thru to the diaper inside.
Can you try getting a few Microfiber Towels, you can find them locally now, but be sure to prewash the colored ones a few times alone or they will dye other things in the wash.
2 microfiber towels are the same absorbency as 2 prefolds, and WAAAAAY thinner too.
Also is baby at a growing spurt age???
Many babies get to an age where they will suddenly output a lot more in diapers, especially at night.
I used an Alexis Featherlite pull-on cover over any diaper I thought needed a little help at night. Those covers NEVER leak!!