Font substitution even though font is present

I've been doing a civil engineering job & I created a drawing for the
cross-sections so a co-worker could annotate those while I worked on
the plan & profiles.
I've been using a font called romansx.shx which I got from
cadalyst.com. It allows you to use %%133 for the property line
symbol.
It's been working fine for years, but my co-worker's computer
substituted simplex.shx for romansx.shx even though the font is
present in the font folder. I didn't realize this, & when I inserted
the cross sections back into the main drawing, it messed it up & now
my computer substitutes simplex for romansx.
This only happens for this drawing, though. If I copy & paste text
using this font into another drawing, it messes up that drawing.
If I change the fontalt variable to romansx.shx, the font will work,
but it has undefined shape errors.
Does anyone have an idea of hat's happening & how to correct it?

Thanks,

Chris