The Seahawks had a poor 3rd down performance to close the Bengals game, finishing 1 of 7 on their final 3rd down chances after going 4 of 6 to start the game.
The Seahawks had two scoring drives where they avoided 3rd down altogether (their field goal drive before halftime and the Rawls touchdown play, which was the first play of the drive). According to my records, the biggest hole in the Seahawks 3rd down game is in the 3-6 yards to go distance -- with a miserable 29% success-rate in this down and distance.
Seattle did show improvement in the Bengals game, posting their best performance from this down and distance, and going 50% (3 of 6) from this range. That being said -- they failed on all three makeable 3rd downs of this distance to close the game.
Here is a table showing the Seahawks 3rd down performance for the season broken down by down and distance:
As you can see, the Seahawks are not doing that bad from 1-2 yards to go (67%), and in the 7-15 yard range (44%), which is why they have been able to keep their overall numbers around league median (about 39-40%) for the season. Another reason why the 3-6 range is so important is that 39% of the Seahawks 3rd downs are in this area (27 of 69 scenarios). Only 26% of the Seahawks' 3rd downs are in 1-2 yards to go, and only 26% (18 attempts for each) are in the 7-15 yards to go range. The Seahawks are doing a better job this year staying out of deep 3rd downs -- only 6 third downs have been over 15 yards to go (9%).
And here is the table for the Bengals game, which does a good job explaining how Seattle surrendered a 17 point lead late. If Seattle converts just one more third down toward the end of the game, that was likely enough to pull out the win.