The game won't spawn more shells if there are more than a few on the ground already.
So, please pick up any shells that are on the ground, even if they are on the ground because you put them there. In this case, the floor of your tent counts as "the ground", too. Keep the shells in your backpack, storage, or a chest; or donate/sell them.
This means that you shouldn't decorate the island with shells. One of the bears you meet will eventually sell you shell-themed decorations--please decorate with those instead since they won't interfere with spawning.
A max of about 20 shells can spawn per day.
They won't all spawn simultaneously, so pick up the few that you see now, then come back some hours later and hopefully there will be more.