Check your times. I noticed an issue where I put in a current time and hugo server doesn’t build the page because of the difference between summer time and UTC. You can also tell hugo to build future posts which might help.
Welcome to Hugo, I’ve only recently joined the crowd myself, also coming from WordPress. It can be quite a steep learning curve, especially if you’ve not used a static site builder before.
@TotallyInformation is right. You should also check if you have the timezone you wish to use set in your .md files. If those times are in UTC then your server recognizes that and might not publish posts that are not yet published in his own timezone.
you can run hugo --buildDrafts to see if that will publish the missing posts.
The hugo theme directory has plenty of simple themes. Just read all information in the readme files of the theme you chose, some have additional steps to be configured properly.