Skill tags are usually set up once, generously, and then never audited. That is how they stop working.
Why it matters
Tags decide who may take which stop. Too broad, and they decide nothing — everyone is eligible for everything, so the wrong person turns up and the visit has to be redone. Too narrow, and one technician becomes the only candidate for a whole class of work, which looks like expertise and behaves like a bottleneck.
The cost of a bad match is rarely booked anywhere: a wasted drive, a second visit, and a customer who was told someone was coming to solve the problem.
How you notice it slipping
Every technician holds every tag. Or the specialist is late every week because all specialist work funnels to one person regardless of geography. New hires inherit a tag set by being copied from someone else.
What to do
Build tags in layers: what the licence allows, what the person has been trained on, and what this site specifically needs. Then audit them the way you audit routes — once a quarter, against who actually did what. Tags drift as people grow, and a tag that is a year out of date is worse than none.