The gel refill is not defective. The gel backs up in the plunger because you need to remove the gel tube from the probe at the end of each day and cap it.
When you leave the gel refill in the probe, gel hardens both inside the probe and on the gel tube. Capping the probe is not going to prevent the build up of excess gel in the probe.
Clean the probe thoroughly at the end of each day and remove the gel tube and cap it with the cap that came with
it until the next time you will be using it. This will save you the additional costs of buying extra gel refills when you still
have a half tube or more remaining.
Please see this Youtube movie for proper Tri Electronic Gel Tester maintenance.