Bought some new jumpers a while back. Only used a couple. A few days ago I pulled a couple out of the bag to test some voltage dividers and found the jumpers did not work. With some, corrosion over time had made the connection between wire and alligator clip high resistance. Others were just not wired correctly.
One needs to check this stuff you order on line, not just because it may not work, but in this case a couple I was using caught fire!
Note that these worked at first, but went bad over time, making the danger sneaky. Check them before use, and if the connections are dodgy, you might want to solder them while they are new and fresh.