How do I spot problems before they turn into callbacks?

You can do the routine maintenance fine. What still trips you is seeing the trouble before it becomes trouble, catching the thing that would otherwise bring you back on a callback. That foresight is a higher skill than the routine itself, and it is the mark of moving from good to expert. Not having it yet just means you are on the way.\nHere is the shift in thinking. Fixing what is broken looks at the present. Preventing failure looks at the future, and that means learning the early signs, the small wear, the slight heat, the faint sound, that come before a full breakdown. Those signs are almost always there if you know to look. The experienced hand is not psychic. They have simply seen enough failures to recognize their opening act.\nSo start building that library on purpose. Every time something fails, on your work or a colleague's, ask what the earliest warning sign was and whether it could have been caught sooner. Write those signs down. And on your routine visits, add one habit: do not just service what is due, spend two minutes looking for what is about to go wrong. Look, listen, feel. Foresight is built from remembered failures, so study every one you meet.