I wondered the same about the shell tiara. She has worn it once as Princess of Asturias, so it's likely that she had access to the tiara back then. But she never repeated it, neither before or after the accession to the throne. It doesn't seem likely that Sofia was sharing the tiara back then, but has since decided to keep it to herself. So maybe that's another one that is not to Letizia's liking?
I'm not sure she had/have access to the Shell Tiara, for most of her Tiara events (2008-2011) Sofia wore the Floral Tiara and Letizia the Prussian. maybe its just me but i guess the shell is "more important" then the floral and usually sofia wore the more importan tiara then letizia so it was unlikley to see the shell on letizia while sofia wore the floral.