Some help with what kind of bike/brakes they are might give us advise.
If you’ve ridden in the rain, sometimes build-up of gunk on the rims and brake pads can affect how well the brakes work. Cleaning the bike can help… avoid spraying any bearings directly with the spray and use a little dish soap with a soft brush to help get gunk off. I also have some wheels that seem to like it if I clean the braking surface of the rim with some rubbing alcohol on a rag. Take a clean rag, squirt some rubbing alcohol on it, then rub it against the braking surface of the rim. You’d be surrpised how much black gunk comes off the rim.
Hope that helps a little.
Pete