It could be many things, but I would start by doing this:
Go to Appearance>Customize then find the “Homepage Settings” option and click there. You will find the option for “Homepage” and “Posts Page”. Set the “Post Page” to “Blog” from the dropwdown.
I have a feeling your issue has to do with either Menu setup, or categorizing, but you should try this first and let me know. Good luck!
Cheers.