Several things could be at work here. First of all, the clock relies entirely on changes in atmospheric air pressure. As a way of forecasing the weather, it has some idiosyncrasies. Find out more in the instruction manual, in the chapters on ‘How the clock works’, and ‘Forecasting the weather with a barometer’.
You may find it reassuring to check the recent air pressure changes in your location which you can do at this website. Simply enter your location then go to the bottom and click 'time machine'.
Here you'll get an indication whether the local air pressure has been generally been rising or falling. For example, at Bramwell Brown HQ, we very occasionally see the clocks producing pessimistic forecasts, even if it's relatively bright outside. When checking this website we soon see that the last 24 hours or more has seen drops in pressure from a previous high - something that in usual circumstances would bring rain - but not on this specific day.
It could also be the batteries. Have the forecasts only just become erratic, after a period of sustained good use? If so, it might be worth replacing old batteries with new ones.
If changing batteries doesn’t help, it could be something to do with your location. Our clocks are designed for the quintessential climate of the British Isles; elsewhere, changes in air pressure do not result in the same weather. The altitude of your weather clock also affects its accuracy, because atmospheric pressure reduces the higher you go.