Daf Yomi - Sukkah 19 - Careful where you Sleep

The Gemora makes a distinction between ineligible s'chach less than three tefachim and open space less than three tefachim. The Gemora states that they both combine to complete the minimum measurement of a sukkah, however one you can sleep under and fulfill your obligation and one you cannot. Rashi learns that one can sleep under the ineligible s'chach but he can't sleep under the open space.

The Taz in O"C 632:4 explains the distinction by saying that open space is much more noticeable than ineligible s'chach and therefore one cannot sleep underneath the open area.

Tosfos asks on Rashi that if one cannot sleep under an open space area even if it less than three tefachim, where can one sleep? It is almost impossible not to have a s'chach with some opening?

The Rosh answers that this is not a concern because air space is only a problem if it is running across the entire length or width of the sukkah and that a person could be careful to avoid.

The Reshash is baffled as to why tosfos didn't answer this himself when Tosfos on the previous daf stated this in regards to the opinion who holds that the principle of lavud does not apply in the middle of the sukkah, we are still not concerned about pockets of air space because it is not running across the entire sukkah. Why did Tosfos choose to ignore this here?

There are Acharonim who attempt to answer this question.