Something to look at.

All of our bikes had linear pull brakes. We have had days when the bikes just did not seem to stop as easily as other days.

When our bikes sit unused for any length of time the aluminum rims form a thin film of oxide on the surfaces. This is worn off on one or two applications of the brakes.

I did find that after long periods of use the brake pads take on a glassy or glazed surface appearance. I then take a piece of 400 grit wet or dry paper and go over the pads in 3 or 4 light passes to break the glaze. The glaze reduces friction between the pads and the rim. Breaking the glaze increases friction and improves braking ability. Some of the pads glaze quickly while other pads do not. Depends on the batch of rubber that was used to make them.