Sidebar next page error in FortySecondsCV LaTeX template

Developer here.
Do I understand correctly: You filled the sidebar on 2nd page already (presumably via addtobacksidebar), added even more sections there and expected them to be visible on a 3rd page?

If that's the case, then the solution is to manually create a 3rd (or in general n-th) page and add a new sidebar environment, which can then be filled as usual:

newpage

begin{sidebar}
  sidesection{Another Sidebar Section}
  % sidebar content of 3rd page here
end{sidebar}

cvsection{Another Body Section}
% body content of 3rd page here

Letting a sidebar overflow will not automagically lead to new pages as would be the case for regular prose text in the body of a simple LaTeX document. You can think of sidebars as "boxes" of page height and some pre-defined width which are restricted to the page where they have been "created" (using the textpos package). You can find more information on specific layout elements in the official documentation.

Note: Posting your question directly in the corresponding GitHub repository as an issue will raise the chances of a prompt answer. ;-)