Ipcs reports only System V shared memory. You must be using Posix shared memory. Apparently these are two completely different mechanisms. Posix shm is shown under /dev/shm.
I've had the same issue, and the accepted answer is not satisfying enough as it is not explaining why this is happening. I found the answer in another similar question:
i cannot see the shared memory created by the boost::interprocess via shell command ipcs
"As far as I'm aware, ipcs reports only System V shared memory. POSIX shared memory is a separate area, and has to be queried separately. On linux systems, check /dev/shm."