(David Coggins) The kiln would need to be firing very unevenly in heat distribution, and I
assume this was the case to require fitting of a three zone controller in
the first place.

(Davis Salks) Yes. When running full tilt, my final temp on the bottom
zone is around 35F cooler than the middle, and the top zone is around
20F cooler.

Also the elements
would need to be slightly low in power dissipation for the kiln size, either
through poor element design (a common problem)

I agree with this

or age.

I often hear coments about element age as related to poor performance.
This confuses me because older elements actually run hooter than new
ones. This is because their electical resistance decreases over use.

kiln was just managing the required firing cycle before the fitting of the
new controller.
After fitting the three zone controller, the top set of elements will be
cycled off some of the time to try to keep the top in line with the
temperature in the rest of the kiln, while the middle and lower elements are
running at full blast . The net result is that the overall power dissipated
during the firing is down enough to slow the firing cycle, or even prevent
the completion of the cycle.

You've hit the nail right on the head, Dave!

Many kilns are so poorly designed that they have very little reserve power
available when new, and after a lot of firings they are struggling to reach
the end of the cycle.

A couple of questions for the original poster. What sort of kiln are you
using?

Evenheat, 55 amp, 6 coils, inside 29" dia X 27" high, outside 34" dia X
32 high

Did you have trouble with uneven firings?

Yes, actually that's why I thought that the 3-zone would help. But I
really never considered that the kiln would not have been able to keep
up. I still don't really know how I could have assessed my kiln's
performance for this application.

technician for many years, and I often had to redesign elements for tall
toploaders to compensate for very uneven firing.

I am open to that.

Presently I am going to backup the lid on the outside with 1-1/2" of
vermiculite board and also add 1" of ceramic wool to the floor inside,
hoping that the added insulation will slow down the cooling somewhat in
these areas.

After testing, I will let you know what the resulting temp diffs are.

Thanks

Davis