Obviously, increased bus capacity would help. The waits are very aggravating. But more buses means lots more $$$ which AA isn't inclined to spend. There should be enough buses so that a bus leaves about every five minutes or as soon as it's reasonably full, if sooner. And if only a few passengers have boarded but the bus has been there for five minutes - it's time to roll.
Long term, of course, the optimal solution would be for AA to be able to use Terminal 5 as an RJ terminal once the mid-field terminal is completed. Delta and United have been able to use their main terminal spaces for RJs for a long time now.