If the file is truly an Addin it will not be visible. Sounds like the file
is a normal file that happens have an xla extension.

In the VBE, in the Project panel (if necessary Ctrl-R), select the
ThisWorkbook module in your project. In the Properties panel (if necessary
F4) Ensure  IsAddin is True. Close all module windows and collapse the tree
in the Project panel.

Save the file with the icon in the VBE menu, or with the project name still
highlit, in the Immediate window (Ctrl-G) type
thisworkbook.save
and hit enter

Actually before doing the above, you say sometimes the addin is hidden but
other times not, I wonder if you have two similarly named files in different
locations, one truly an addin but the other not. Might be worth searching
your HD for duplicates. Rename the non-addin as .xls

Regards,
Peter T