Hi, I think I had a similar problem and found a way around it.
For me, it seems to happen when one family has several variations in several files, and you try to install each file separately. The first file install works; every other one gives a dialog saying "already installed". If you say "Replace", it still isn't
installed.
What worked for me (as Administrator):
- Go to folder Windows\Fonts and look for the family name. Delete any items in the font family which are already there.
- Open another folder with the font files you want to install. Select all of them. (For me, 18 Lato*.ttf files.)
- Hold control key, click and drag all the files into the Windows\Fonts folder to copy them all at once.
- (You MAY get the "already installed" dialog. If so, select "Replace" and check "Do this for all remaining items".)
If you then look in Windows\Fonts, you should see the family name as a single "stack" icon. Double-click that icon and you should see a bunch of individual icons for each variation.
NOTE: Fonts are sometimes grouped strangely.
All the "Lato" fonts show up in a single "Lato" group, except for "Lato Hairline Light" and "Lato Hairline Light Italic" which show up in a "Lato Hairline" group. It must have something to do with how many words are in the name.