Good Power Bank - but the solar function disappointed

Compared with the experiences of a Power Bank ZNEX® V'bee also EasyAcc® can convince. Reliable, short loading times, quick recharge, practical mounting option.
The solar charging function, a major reason for the purchase, however, was disappointing.
After 10 days (upright position, south side) was not recharging recognizable. Same battery level as before - about 25%.
Because such statements were also made by other Rezessenten similar solar equipment, it probably is not in the device by a random defect.