... have you any thoughts why the software wouldn't pick up on the other images that are very similar in my collection?
I have no information about what the software's rules are, but I did a little experimenting with a series of mine. What I noticed is the following:
1. The series shown is not dependent on the search terms you use for an image - a given image has its "series" (at least the 5 thumbnails shown) however it is found
2. Looking at the first 10 keywords for the images I experimented with, all of them had some keywords in common but not in the same order. I put the most significant terms for a particular image up front and the groupings suggest to me that keyword order, not just presence or absence, make a difference.
3. Clicking on "See More" gives me varying numbers of items in the series, again suggesting the keywords are being used to collect items. Items that are not part of the series (as I see it or intended it) show up for some images but the content is related-ish
I think keywords are the area for you to focus on - making sure you're keeping them accurate to the image content, but ordering them consistently across the series