Lubing the spring should fix it. If not, you can always replace it with a spring from a key you don't really use.
Always lube your springs, even if you don't lube the rest of the switch!
You can use Victorinox oil or a high-viscosity silicone oil. Put all the springs in a plastic container with some oil and shake until evenly coated. If springs are stuck together, make sure to twist them apart and not pull, to avoid damaging them.