Doorbell 2 solar charger

I bought a solar charger over a month ago after noticing that my battery last significantly less than before i.e. it now last for about 2 weeks where before it would last about 2 months. The solar charger status shows it is connected but very often it states that the sunlight is insufficient to charge battery (this is in stark contrast to the separte solar unit I have for my eight deck lights that come on without fail). On occasions the charger status shows it's charging but there is no increase to the battery power level. In fact, it decreases. Can anyone help!?