The further you are from the equator, the longer it will take to fully charge the WakaWaka via the sun. Hence, it requires some patience to charge it via the sun in Europe.
The best way to charge it is by putting the solar panel face to direct sunlight and without anything blocking it. Try to turn the solar panel along with the sun during the day so it will always face it.
If you put your WakaWaka behind a glass, it will lower the charge efficiency drastically.
To fully charge