A part of the light emitted by the sun consists of the visible light that we can see with our eyes. Beyond this visible light, sunlight also contains components that we cannot see, for example ultraviolet (UV). Your WakaWaka charges with this UV-light.
Please note that daylight is not sunlight and therefore is not UV-light.
Depending on the season, the weather condition (fog, cloud), altitude, dust in the atmosphere, the distance from the equator and other variables, it can be more difficult to fully charge the WakaWaka with solar power. For example, an hour of sun in the winter gives less UV-light than an hour of sun in the summer. When it is not very sunny or when the WakaWaka is behind (double) glazing, the charging efficiency decreases by 70%.