So are are you able to get good solid pressure on the rim from the brakes and it's just pushing through them or is it not getting the clamping power needed to squeeze the rim? If it's the former then there isn't enough friction between the rim and pad. Lots of answers for that here already, but I'd just add that using a file to remove a MM or so from the brakes pad surface is oftentimes enough to get rid of the slick, dried out layer that forms over long periods of non-use. If it's the latter then it means it's out of adjustment or something is broken. If you can't dial it in and the rim and pad is fine, I'd look for something broken or flexes badly when you grab the brake. Sometimes even just a cable housing can fail in such a way that allows it to compress way too much. With the info given, though, I'd guess it's a rim/pad issue.