You're all forgetting what FTG essentially is... It's just a summoning jutsu in which Minato summons his body over to another location that he has marked with the summoning seal. As we have all learned from Jiraiya when he taught Naruto, summoning creatures, even if it may be small, requires a lot of chakra. In a real battle, Minato uses FTG quite often, meaning, every time he summons himself, he uses up a lot of chakra.
Kakashi can summon those dogs, but think, how often do you see him actually summon them? And when he does, how often is it that he summons them IN battle? As far as I remember, he only summoned them in battle twice, once for Zabuza and once for Pein. The reasoning is because he cannot prolong a battle after he summons them due to the amount of chakra they need to be summoned and for the fact that Kakashi already stated he has a very poor chakra reserve compared to that of Minato who had a huge amount of chakra. Throw in the fact Kakashi has to use the sharingan too means he'll be out of chakra within minutes. As we've seen in Tobi vs Minato, you can't rely on just a few minutes against a powerful enemy.
Simply being, whether Minato tried to teach Kakashi or not, I'm sure Kakashi tried to learn it anyways and realised he's incapable of performing FTG to it's full potential. Who knows, maybe Kakashi actually can perform FTG, but like I said, due to the amount of chakra required, it may be a trump card for him.