I have a similar experience managing a machine shop.  After much effort, the team reduced a cell set-up from a typical 3 hours to consistently 15 minutes.

We were proud strutting around as "accomplished our goal" when the GM of the divsion invited the Shingijutsu representative visit our "success".  His two comments deflated our egos...he said we had "the look and feel of improvement, not the heart and soul".  and that we need to "remove the 'teen'..."  and promptly left.

It took some time for us to understand his comments and realize we weren't fully looking at our system.  A few months later, we had revamped the cell and capable of ZERO set-up time for 90% of the variety that went through the cell (over the following year or so, we even doubled the variety we produced throught he cell!). The remaining 10% could have been accomodated, but we chose not to spend the money to do so...the 10% we could change in under 5 minutes (and met the delivery requirements outlined below).

One of the key drivers was changing our goals and metrics.  Instead of monitoring set-up time (internally focused and really only concerned us), we set 1 hour windows (arbitrary time that made the follow-on processes comfortable...e.g customer focused!)and Kanban bins were sized to fit the hour time frame (again...agreememt with the customer).  When the bin was too large (not empty in 2 weeks...another arbitrary set point kinda linked to the company's inventory turn goals), we shrunk the bin size and subsenquently drove set-up improvement, cycle time improvement, error reduction, etc.  if the bins were "too small" (e.g. empty in less than an hour), we cut the "time to fill" by 1/2 (another arbitrary number..easy to do the math!)..this in turn drove the same improvements in set-up time, processing time, error reduction, etc.

What we transferred to the other cells, was the the change in goals, metrics, and the feedback loop.  Certainly some of the learnings about tool selection, fixturing, programming, etc. followed, but only in the context where it was applicable and needed to improve our new "customer focused" metrics.  we still measured set-up time, but only as part of our 'system" and a component of a modified "standard work" playbook for the cells.

the shock to our shop that we had done eerything well, but not yet finished was a difficult leason to learn.  We were well trained in the tools and thought we had accomplished much.  Once we became educated, we realized how far we had to go...