I am seeing another occurrence of something that I had seen before, where a
given
antenna design that should be workable, simply won't tune in a given
design.

Might you be exciting higher modes when certain dimensions allow. eg
various portion trying to become 1/2 or 5/8 or ... wavelength or whatever
under only some conditions.
[I suspect that that question may be so simplistic in your application as
to appear as ignorant as it is :-) ]

R
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