The black "bombs" were facing the other direction in a black room. The military was running the show. Louise and Hawkeye were interested in the Aliens not taking inventory.
Also, the Aliens had to wait until they were both present, so that they would realize the importance of the words which were not random at all.
Excellent movie, gonna pre-order it Blu-Ray.

Or, why didn't Lois Lane's "nose for news" sniff out something wrong?
Seriously, though, the aliens have already lived this memory so knew about the bomb, when it was to explode, and at just what time to push Hawk and Lois out the chute. For them, this was a 'memory.'
There is a similarity wherein Abbott knew Costello would be mortally wounded but chose to accept this sacrifice to ensure their 3,000-year-old future remain intactmuch in the same manner Louise chose to accept her daughter's death knowing that was her future.
The explosion was inevitable so Hawk wasn't supposed to find the bomb.

To add dramatic tension to a film that was the action equivalent of the vagina monologues.

The bigger question is why didn't the aliens notice it? Or rather, why didn't they act on it quicker instead of waiting 'til the last second?

Your "Hawkeye" tickled me. Sorry

The bomb thing is is really troublesome. So we see the bomb just standing there like a chair in an apartment, the clock is ticking and they don't see it? They don't see there's an object in the ship which shouldn't be there? That's stupid.

Nice catch