Some things to look at. Are the jaws pivot point loose? That might cause the cutting edges to be too far apart and the wire is simply getting bent between them. Are the cutting edges sharp? If they have been used a lot, they might simply be too dull to cut and they are just bending or mashing the wire between the jaws.
Also, the wire you are attempting to cut is multi-strand. Multi-strand generally has to be worked in smaller "chunks" with conventional wire cutters or undersized wire cutters. Your probably cutting some of the outer strands but not getting through all the strands and the whole mess is just getting pinched between the cutters. You can try to work the cutters around the outside strands by rotating it as you cut enough to eventually work your way completely through which is how I do it. Or, you can try separating the cable and slip one of the jaws through and cut fewer strands at a time. Usually this is more difficult than my first suggestion because there's a vinyl covering on the outside, though that can be stripped away first, just more work.
Mike