I too have had this happen in the winter. The cable housings get water in them - when the water freezes and makes the cable very, very stiff. When you pull the lever you can overcome the stiffness, but the return springs inside the drums are not nearly strong enough to pull back through the resistance.

New cables usually do the trick - I've never had much luck reviving old cables once the seals on the end have allowed water inside - they get stiff from rust sooner or later anyway. Get some new cables and try securing the little rubber seals a little better - like a little wire around the base to make sure they don't pop off, perhaps a little tape cone around the tip to pre-wipe the cable or even cover up the section that goes in and out.

But basically they just need to be replaced every once in a while.