Spell Checker - problems with diaeresis

To get around the awkward diaeresis in the names "Ykspetäjä" and "Häkkinen",  I set "Ykspetaja" to be replaced with "Ykspetäjä" and "Hakkinen" with "Häkkinen" in the User Words List. However, "Ykspetaja" doesn't work while "Hakkinen" does. What's going on? Curiously, if I set "YYY" to be replaced with "Ykspetäjä", it works. Why won't "Ykspetaja" replace with "Ykspetäjä"?