Last month, a new patch for Ghosts had a glitch where players couldn’t plant or defuse the bomb in S&D. That original glitch was fixed as hot fixed, but then a couple weeks ago, IW released another patch for Ghosts and brought the glitch right back into online play in Ghosts.
Today, Infinity Ward has finally fixed the glitch in online play and private matches. You should be able to plant/defuse the bomb normally in Search & Destroy.
UPDATE: The S&D bomb plant/defuse issue is now fixed in #CODGhosts online play. Thank you for your continued patience!
— Infinity Ward (@InfinityWard) August 14, 2014