A few things come to mind first up...
Wool covers need to be lanolised to be water proof. They also need to fit correctly so no gaps holes or loose area where they can allow leakage.
The bbh appies need to be washed about 10 times to allow the bamboo to reach max absorbancy. So depending o. How old you nappies are they may not yet be attheir full absorbancy. If the nappies are well used or are second hand they may need to be strip washed. This will remove any detergent and dirt build up in the fabric fibres and increase their absorbancy. You can do this by washing on HOT and using a few drops of dishwashing liquid and a few drops of lavender or tea tree oil.
How do you wash the nappies normally? Using too muh or the wrong dtergent can affect their performance.
Again the nappies need to fit correctly so no gaps around the legs or tummy etc....

Failing all that it may be time to move up a size in nappy or try a different brand fit or most likely a dedicated night nappy like the BBH night nappy or Sandman. Personally if she is that heavy a wetter you need to get a dedicated night nappy as the are generally twice as absorbant as a regular fitted nappy like you are using now.


HTH and let me know how you go