It doesn't happen often, but it does happen. Sometimes nappies leak for simple and fixable reasons.
If you are having trouble, here is a quick checklist to consider.
Are the nappies new?
If yes, then the nappies may have not been washed enough to remove the
fabric softeners used in manufacture that greatly reduce the absorbency of the nappy. You need to wash the nappies twice
again, preferably in hot water, with a regular amount of regular washing detergent. Most nappies are usable after two washes
but take 5-7 washes to reach full absorbency.
If the nappies are not brand new then ...
Is the cover the right size and being put on correctly? The cover should be snug but not tight
and the whole nappy must be covered. The cover should not be too big or too small.
Is the cover worn out? Covers last around 4-6 months of full-time use -
after that they may start to lose their waterproof properties.
Is the nappy saturated ? You need to change the nappy more often or start using a booster.
If the nappy is too snug to fit a booster inside, lay the booster outside the nappy but still under the cover.
This is an especially good trick for night nappies!