help with wool covers

I was all excited about using the new night nappies I bought, but so far I have had nothing but troubles with the wool covers.
They have leaked each night. It is compression wicking and only at her back. She sleeps on her back and stays in one position all night. Her pj's and sleeping bag are damp at her back in the morning, and have that urine smell to them.
The nappy doesn't feel really full, if that makes sense, I think it could take alot more.
I asked my mum to knit the covers and gave her a couple of patterns from the net. The ones she has knitted seem okay to me, the only thing is when you hold them to the light and stretch them a little you can see through them. Is this normal or are they mean't to be thicker / more closely knit?
I have lanolised them, and I am re-doing them now hoping that maybe it can have a cumulative effect.
So,
How thick are wool covers meant to be?
Does anyone have a good, reliable pattern for ones that work?
Has anyone got any other suggestions for what I am doing wrong?
Thanks:)

Are you sure the wool is 100% wool? A blend with acrylic won't work, even with lanolising.
The thickness of the wool needs to be relative to the size of the needles also. ie using 8ply wool with bigger than 4mm needles will make it too loose and contribute to leaking through too.

defiantly 100% wool, though I am not sure about the ply or needle size. Mum said she followed the instructions. Should you be able to "see through" the cover? She has knitted two, would using both together help?
The one that has just finished an overnight lanolising seems heavier now, maybe I wasn't doing in right the last time?

I used the two covers together last night after re-lanolising them, and success. The outside of the nappy was damp, but her clothes were dry. She did have the worlds biggest bubble-butt, so cute! When the weather improves and I can dry easily I will try and use just one and see what happens.:clap:

Forget that last post, I had her in both covers last night and her pj's and sleeping bag were damp:doh:. So frustrating. Anyway, just bought a BBH wool cover, they are on sale so I took that as a sign. I will be really annoyed if that leaks.