Image series

It could. But you're asking us to guess based on zero information.

See below about 'similar content' and also 'variations in vector graphics.'

• https://helpx.adobe.com/stock/contributor/help/how-to-submit-distinct-content.html
• https://helpx.adobe.com/uk/stock/contributor/help/vector-requirements.html

Adobe has really clamped down on similars, and that seems to have become the most often used rejection reason. If there are really only minor differences between your similar images In the series, don't submit all of them. You can be considered to be spamming the database, which can result in account suspension.

Sure. 

HOW varied? I'm lucky if I have 1 of 3 selected.

Who knows?

These days, nothing can be said for sure. Even some unique images face the 'similar' issue