Its just stupid to say that he did not know he had it. Kakashi and obito unlocked it at the same time, however kakashi fainted due to his low chakra levels whereas obito is an uchiha with senju dna. at that time it took kakashi a lot of chakra just to use the normal sharingan, the mangekyou was off limits seeing as he fainted just by awakening it.
after facing itachi's mangekyou he decided to improve he chakra control during the timeskip and has now improved it even further, that why recently kakashi is now able to spam kamui. if u had watched closely all his improvements during the timeskip were in order to be able to fight itachi (mangekyou, faster hand seals etc.)
remember when we first thought sasuke awakened his sharingan against haku, but after a flashback we saw he awakened it during the uchiha massacre against itachi. but looking back to the haku fight you actually notice he already knew he had it.