For Conjure Dremora Lord aim at the floor (the crosshairs will be red if your aiming out of range) and make sure the spell is charged, also according to the bugs list they can appear invisible on the 360 (could be for other systems too) opening and closing the menu might fix that.
I also cant cojure a dremora lord. Stopped working after I made a glitched potion to make my conjuration spells last rediculously long. Reloaded a previous save, still doesnt work. I may have broken my game.
You might want to try conjuring other creatures for a while to see if that works at least. You could also try getting you conjuration to 100 and then making it legendary in case that helps reset whatever the issue is.