Problem hanging ceiling lamp with cord set (from Ikea)
I have a similar Ikea fixture, but it came with a metal sleeve that tightens with a screw--this part sits below the cap, but this light has a rod (rigid, not the cord itself) for this sleeve to work on.



Did you buy it new? If for some reason you didn't have the instructions and therefore couldn't verify if a piece was missing, you can find those here: http://www.ikea.com/ms/en_US/customer_service/assembly_instructions.html I know you can order replacement parts from Ikea, at least if what you need hasn't been discontinued.

Assuming you have the instructions and all the pieces, I think your best bet would be finding a rubber O-ring (or several) which match the cord diameter, and sliding those up to keep the cap up....or using a twist-tie painted to match the cord. Once you have the cap up reasonably flush with the ceiling, you could use clear silicone caulk around the rim of the cup, which can easily be peeled away later if you're renting.

Finally, is the hook all the way in? Perhaps screwing it up further into the ceiling would make the cap go higher.

Best of luck!