Minor Blade Raw Material Prospection bugged?

[buschdk]

I got this option and expected it to help me stuff that could be used as blades - normally that would be shells. However I have never been able prospect any resource with this option. I can stand right in front of a place where I have previously extracted choice shells and get told that there are none of the specified raw materials!?!

Is this bugged or is there something I am missing?

-soeren