This may be unrelated to Start10. One way to find out would be to select the Windows 10 menu style in the Start10 configuration. That uses the native Start menu.
There are a number of reasons why the taskbar may fail to auto-hide. Here's a search that will find a number of articles on the topic:
https://www.google.com/#q=windows+taskbar+autohide
Some of those are for older versions of Windows but are still relevant for Windows 10.
