Hey @Juaire! The power being supplied to your Doorbell 2 certainly sounds sufficient. The best first step is going to be to check connections as well as wiring to ensure it is not too thin or causing resistance.
Being that it worked recently, keep in mind that a difference in usage, number of events, wifi signal strength, and even temperature can cause a battery to drain more than normal. Check out our Community post about battery drain for more tips on optimizing battery life. I hope this helps! 