Are you sure? You need to be under 80%, which with an inventory of 80 means you need to have less than 64 items.
If that's not the problem, it's lag. Just keep trying to accept it and it will go through eventually (you might want to click "Acquire" or whatever it says then go AFK for a few minutes).
I have 2 characters on Shilien, and sometimes I get items wrong from Dimensional Merchant (DM).
f.e. my
1st char is - Tramis 40lvl,
2nd char is - Tramis2 1lvl.
I claim 40lvl reward for my 1st char, but my 1st char didn't get it, 2nd did..wtf? :D