Occasionally I find that I can't use consumables while going through a level. Logging out then logging back in only works half the time. This bug has happened three times (including today), on both "normal" and "advanced" settings. It also happened during a prestige mission. I honestly have no idea what to do.
This is what it looks like:
http://i57.tinypic.com/3145v0o.jpg
If you unlock your HUD, you can click on them to use them, but if you have it locked you can't click on them to use them, you can only use the key bindings you've made to use them once it's locked. Hope this helps
"EDIT" Click the little lock that is on the mini map in the top right corner to unlock the HUD, or just use "Q" if you haven't bound it to something else already.