I had no luck with getting a perfect cow berry from a dragonfruit grafted to snapdragon - more than an entire year in, I got one dragonfruit and then nothing but snapdragons. Finally, I tried the opposite, splicing snapdragon to dragonfruit, and got both dragonfruit and several cow berries. So, you might try splicing the opposite way, or splice another plant? It shouldn't matter which way you go, but there was clearly something not right with my first try. At very least, it should have been producing both snapdragons and dragonfruit.
