Yes, the census taker could skip them. It happened to my grgrandparents in Everett MA. They are there in 1900, 1910, 1920, but missing in 1930 even though they lived in the same house during that time. The enumerator missed their address all together!They are enumerated in that same house in 1940. The census taker just skipped them in 1930. maybe they weren't home when he first came round and he forgot to follow up with a return visit. But they lived there and are not enumerated. So it does happen...not often, but it has happened.