It's a random thing. You're just having a run of old-fashioned bad luck...the RNG doesn't like you right now!
Another idea: Not all the crabs carry shells, and those that do, don't always drop them. BUT: you can pickpocket them for shells even if they won't drop them. Get a character high up in pickpocket to attempt it on them before they drop the shell.
ALSO: not all characters need to go through Collasium. All you need in there is the 3 tablets. There is a lot of good treasure in there, though, so you may want as many inventories as possible to cart it all out.
Make sure you take a character with the 6th level "Breath of Air" spell too! Even with the breathers, you can run out of air if you're not careful.
-Sazerac
[This message has been edited by Sazerac (edited 05-02-2001).]