Hi @Boilerbabe! Direct sunlight is the number one variable to check, and it sounds like you have. Beyond ensuring plenty of direct sunlight, I recommend also not having too many motion events, ring events, or lengthy live view events. Wifi interference can also drain battery as the Doorbell must fight to remain connected. This can be checked in the device health section of your Ring app.
It is possible you are receiving the charge from the Solar Charger, but the battery could be draining quicker than the charge can apply. Checking the above variables should certainly improve this for you! 