It's amazing how when you are wading into a new application that is so unfamiliar that may you inadvertently dig holes against your progress.
I understand what you are saying and did notice one photo that did have a caption displayed, but it was from a 5 years ago so I must have used the Windows file editing, as you mention, to add it there and then.
In the meantime I've been hunting for a freeware tagger that is easy to use, and, effective. Its XnView which is an Irfanview look-alike but hopefully effective. (not tested as you'll read next)
So I start off by going into each photo and removing all the metadata I placed in the 6 test photos last night (that didn't work). Then I edited in new title and description .
Next I started up JBPro and was AMAZED to see a simple yet hugely welcome caption editing dialogue and editing option.
Clearly Irfanview captions both aren't recognised, as you say, but also stop that caption facility in JuiceBox from even appearing. I never saw (maybe 'noticed') it previously although I saw reference to it in the guide and as you haven't referred to it thought it didn't apply under my installation for some reason. I appreciate the benefit to be able to use IPTC data if it is already exists in the users library.
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I just mention all this in case it helps anyone else new to the application but having similar difficulties.
Good stuff and all's well. Thanks for your help.