The Sora lever has two positions for the cable attachment; one for short-pull brakes and one for long-pull brakes.
It sounds as if you start out with the cable in the right position, and when you pull the lever, the cable slips to the position with more slack.
There is a plastic piece, secured by a small screw, which keeps the cable in the proper place. Is it possible that your lever is missing this piece (see image below)?


This is a brand-new Sora lever, waiting to be installed on a folding bike I'm building up.



Here's the pivot, with the cable attachment point in the "short-pull" position. Note the small screw under a stamped letter "V". It holds in a piece of plastic that fills the slot between the two positions.



Instructions for the Sora levers.

Hope this helps!