Thanks for the reply Progo. Nope, I'm trying to store any of the items in the pop-up grid were all your available stuff is. So for example, any of the nine optional special weapons you could be carrying. I can see that the "store" option isn't even available to equipped items. The "store" button is there for me (on my non-equipped weapons), but it just goes red and does nothing when I tap or hold it. It's a weird one that I can't seem to find an answer for!

Really odd... Only thing I can suggest is rebooting your console :/

Yep it's a weird one! I turn the console off after every session and it's always been like this, so that won't fix it either unfortunately. I'm hoping Bungie have noticed the problem and are working on it..........