During a post-validation survey, you find an area lacking any detectable Wi-Fi signal at all...

Move one or more APs so that indoor propagation results in coverage in that area or install another AP on that side of the elevator shaft

Turn up the power on the three APs closes! to that area

Install antennas with at least 3 more dB of gain on the two APs closest to that area

Replace all the clients in that area with clients having better receive sensitivity