I am looking for IAH-ATH award travel. A while back there was plenty of seats via IST on TK. Now nada. Did I miss something?

Yes. TK opens availability in batches. When it's gone it's gone.

OP is right, the availability for TK is almost completely gone.

I just had an dummy flight I booked earlier during the saver sale from IAH-IST. After dropping those tickets, I don't see any availbility on UA.Bomb for TK on that day.

Does UA have different availability for TK awards? I'm seeing availability on all three ExpertFlyer, AirCanada & ANA sites but not seeing anything on UA.

Also IST-NSI-DLA shows Y award space on the day I'm checking, but IST-NSI does not (though it shows on EF/AC/NH).

How soon will Turkish award space be available on Mileage Plus?
 

I've been looking to pull the trigger on a trip next year that would be ideal if I could use Turkish to get from the US to Istanbul.

Presently, Turkish award space from either ORD or IAD is plentiful up to and including May 15, 2016. After that date, there is absolutely no TK award space. Of course, I'm looking for the next week or the first week of June 2016. I've been checking every day for two weeks now.

Does anyone know how often they "update" or add an additional week/month to the UA award calendar?

The wait for availability is killer, mostly because it's so unknown as to when/if/in what quantity it will occur.