To save me typing it all out again can I please refer you to this post – https://simple-press.com/support-forum/sp5-general-topics/sp-not-recognizing-added-members/#p162299 – which covers the same, exact question in some detail.
I am guessing that this would be the reason but if not please do come back and give me some more detail.
As to new users appearing in the new user lists – afraid I have no idea why this should be the case in the WP user list which is not something I have heard of happening before. However, the SP new users list at the bottom of the forum display is cached and updates – by default – once an hour. So hopefully it will catch up. If it does not then it might mean your WordPress CRON system is not working. If this is the case come back when sure and we can try to get it moving again.