Something similar happened to me and it took 5 revisions and an External Ventricular Drain to figure it out. My NS, too, thought that I was cured from my hydrocephalus because my ventricles weren't very big but yet I was having symptoms of malfunction. He hypothesized that maybe I didn't need the shunt and that it was giving me headaches because it was in the way or something. So he took it out, and in doing so, pretty much discovered the source of the problem. The distal tubing tore because it was so old and gross and they had to go into my neck to pull the rest out (yuck!). So it probably was no longer effective in draining away the fluid, and that's why I was getting headaches. Despite the evidence, they replaced the shunt with an EVD and cranked up the pressure to see if my brain could absorb the excess fluid. Within hours I had extreme headaches, vomiting and my eyes were bulging out of my head and I had double vision. The NS then hypothesized that (and this is where it might relate to your case) my ventricles had been drained for so long that ANY change in pressure or size of my ventricles resulted in malfunction symptoms although not any of the "full blown" malfunction symptoms (like vomiting and sunset eyes). It's frustrating that NSs don't feel the need to do anything unless there are full-blown symptoms. However, the daily headaches and nausea make everyday life difficult. It's like you're existing but not really living.
The fact that you are having symptoms, your MRI shows that the ventricles ARE enlarged (however minor), AND they have proven that neither shunt is working, suggests, in my opinion, that you DO need a revision. I am not a NS or anything, but I would get a second opinion, if you don't want to go on living with your symptoms. One of the hardest parts of this condition is the tireless advocating that we must do. I hope this was helpful. Take care.
Anna