Due to a bug in Illustrator when dealing with compound paths, you might end up importing incomplete shapes, missing parts or weird overlaps in Fontself that can appear as white lines / borders in your shapes for example.
In such case, clean up your vector shapes:
First, try using the command Object > Flatten Transparency with default settings to clean up a shape.
Else try boolean operations: start by applying the Pathfinder > Unite feature in Illustrator to merge all the shapes for a single character (you should turn on the Remove Redundant Points option in the Pathfinder's options menu to clean up extra points).
Try with Compound Path > Make for shapes with no counter-forms
If you have stroked-only letter with counter-forms, and want to avoid outlining strokes, try Compound Path > Release instead (make sure that all parts of each glyph are still grouped as a single object before importing in Fontself).
Upgrade to a more recent version of Illustrator past 26.2.0 and restart the computer: this can solve some compound shape issues (like different parts of a single letter like
ibeing ungrouped into 2 different glyphs).
If the issue persists, try to select multiple glyphs at once and import them all at once using the Batch button.
In most cases, you should then be able to import your glyphs properly. If not, send us your .ai source file to email@example.com and we will have a look.