This article is from the Fencing FAQ, by Morgan Burke with numerous contributions by others.
Electric jackets can go dead for several reasons, including high
electric resistance due to oxidation and corrosion (usually
accompanied by visible discolouration), broken metal fibres, or
tears in the fabric.
High-resistance areas that are due to oxidation can often be
temporarily resucitated by moistening them with water. As the
moisture soaks up salts and other deposits in the fabric,
conductivity will increase enough for the material to pass the
armourer's check. Sweat from vigourous fencing will have the
same effect. Some fabrics do not rely on conductive fibres, but
rather are coated with metallic powder; these will lose
conductivity when dirty, and require regular washing.
Small dead spots can be "field-repaired" with a paper stapler or
Larger dead areas and tears in the fabric can only be reliably
repaired by stitching new metallic fabric over the affected
areas. If no patch material is available, the fabric from one
dead vest can be cut up and used to repair another (the material
from the back is generally in better shape). Note that large
areas can go dead due to broken fibres in a relatively small
patch. Patching only the region of broken fibres can re-activate
the entire dead area. Careful testing with an ohmmeter will
determine where the dead zone exists. Patches should be folded
over at the edges, and the stitch should overlap the edge to
prevent flaps that will catch points.