Font Book is extremely picky about what it considers to be errors that make font unusable and often does not actually explain the problem. When testing my fonts I sometimes have to scour warnings in MS Font Validator and FontQA to find what non-problem Fontbook’s developers decided represents total font corruption.

The version difference is probably because Fontbook was overhauled for 10.6 to make it a less horrible piece of software but I don’t think that Apple backported some of the bug fixes. It may be that your font is fine and that the problem is just bugs in older versions of Fontbook and that the customer will have to either upgrade to 10.6 or buy a decent font management app. This is one of those circumstances where designers have to educate clients that the actual quality of Apple’s software is far less than what the public perceives it to be.

I agree with you that it's a completely unusable piece of software for professional use, but I find it hard to tell a customer that I can't do anything about it. There's not even an error message when she tries to add it, it just does nothing, with seems weird.

I guess I'll have to tell her to just buy a decent font management software – all the free ones I've seen are less than perfect for professional use.

I use Fontbook all the time and never have any problems. I use it to install all of the fonts I make and they all do just fine. I've been using it for years now and have never understood why so many people hate it.

I also use FontAgent Pro on occasion and that appears to be OK, but LinoType's FontXplorer X or whatever it is called these days never worked for me at all. A real piece of crap IMO.

To close this up: as it always seems with customer service, it was obviously her mistake ;) her font book will not let her install any new fonts at all. Good for me, bad for her.

Two potential sources of problems with OTFs on Mac OS X (esp. 10.5 and earlier) are:
1. Subroutine compression: Mac OS X sometimes fails to decompress subroutines properly. Generate a font without subroutinization and try if that works.
2. Too long glyph names in the font (in total): Mac OS X imposed a limit on the overall length of all glyphnames in the font. However, to hit that limit, your font needs to have at least 2,500-3,000 glyphs. If that's the case, shorten the glyph names as much as possible (use one-letter suffixes for OpenType alternates, and the uXXXX notation for encoded glyphs).

I've been using Linotype FontExplorer X Pro since its very first version, and love it. It deals very well with duplicates (e.g. same-named fonts in various formats), where FontBook gets confused. The only feature I'm still missing from Font Explorer (and having asked for it for many times) is the ability to search within character sets (as in: show me all the fonts that have a glyph for "ж"). You'd think it's absolutely trivial, but I haven't seen any single font management application that would allow that.