Split AC - One room in three not cold

Aloha All, I've got a split ac system, air handlers (is that the right term?) in three separate bedrooms. Everything is fine in two of the rooms, but in the third, the ac blows, but does not get cool. Its been that way for about 6 months now.
What should I check first? If it was a refrigerant leak somewhere, would it leak from the other two also, or does that line have separate refrigerant?
Any suggestions would be appreciated.
Email copies of replies to bill at kennedy.us appreciated.
eKennedy
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Do you know how each air handler is supplied?
Do they have their own outside unit? Do they each get chilled water from a single source? Do they each have their own evaporator unit from a single outside unit (I don't even know if these are made) or do they all get air from a single source?
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Joseph E. Meehan

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