AA themselves can only issue up to 16 segments. Whereas travel agents using SABRE can issue up to 24 segments. Go figure!
CX is the only airline I know that can issue up to 36 segments I think without any problem.
BA's and QF's system can only issue up to 9 segments.
Gallileo (owned by UA and used by travel agents) can only issue up 24 segments as well. It's just hard to squeeze in all the taxes these days.