I'm questioning your soil, it looks like you just plunked it in the ground. I have this one cactus that was supposedly "hand picked from a shaman's garden" or whatever and it is the most sensitive thing to water I've ever seen. It is worse than the brown paper you dry your hands off with in the gas station bathroom. Anyways, it still grows.
My point, if the top is still growing, the flow is getting through the base. If the top is visibly suffering, the base is closed off. You could pull it up, use a sterile knife to slice through live flesh and then put it in front of a fan on low and let it callous over, ultimatly leading to a replant. Or you could just let it keep going if the tip is alive. You have to play these things by ear.
Either way, you need to make appropriate soil. Assuming what I see in the photo is correct, you need to make appropriate soil and your cactus will take off. If what Im seeing isnt accurate, my apologies.
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