Genuine users told they’re spam bots

I participate in a blog hop or round robin at least 3 times a year. This time though, I’ve hit a heck of a snag that I can’t find the answer to.

Some of the participants on this hop cannot comment on my site. When they try, they’re told that they’re spam bots & to contact Admin to resolve it. Well, I don’t know *how* to.

I’ve checked settings / discussions – I’ve unticked “users must be registered”. I’ve asked it to close thread over 200 days old (this latest blog is 3 days old)
I can comment on it if I log out of my site or from a different PC on my own home, but friends in the USA cannot.

I don’t know where to look or what to look for now to resolve it.

Please, can anyone help? I’m using version 4.8 with the running theme Twenty Seventeen.
I have Akismet on via JetPack & I can’t find anything in there to cause this.