Inconsistent problems copying vectors from Illustrator

Hi there, apologies if this has a simple solution or if I am missing something. I am a complete newbie.
I have been following a step by step guide by Ben Dunkle. Very simple and easy, creating vector glyphs in Illustrator and copying them over to the Glyph app. A few characters copied over are not showing in the main ‘font’ tab where you can see all categories. They are also not exporting and just leaving a blank space when typed. I made sure that all characters are universal in size and everything else seems the same in the characters that do work. These include the lower case ‘z’ and the zero and some others so I can seem to figure out a connection there.
Can anyone help?

Sounds like a font cache problem. Read this, please:
Eliminating Font Cache Problems: