Ships are not coming back with Shell rewards

You can collect and accumulate a maximum of 200 Shells. Once you reach this limit, the ships will no longer provide Shells until you spend some of them.

If you send ships while holding 200 Shells, but a new destination unlocks shortly after that and you spend those Shells on a newly unlocked city's rewards, you will NOT receive Shells with the coming ship/s since it was originally sent while you were at the 200 Shells limit.