Board Thread:Theories and Speculation/@comment-27717552-20160513151229/@comment-734582-20160514110825

It's not that Minato was unable to add his Element to Rasengan, it's that he died before he could.

Kakashi mentions he doesn't have the chakra to make full use of Shadowclones, so that's out of the question. Furthermore, Kakashi's limited reserves means that every jutsu counts. Raikiri is pretty much an instant kill, whereas Rasengan is designed to be an incapacitating jutsu. The ability to finish a fight instantly is superior to merely defeating an opponent when every jutsu counts. This is why Kakashi never uses it. As mentioned, given how Kakashi grabbed his arm, it's not clear if Rasengan was easy to do for him, in fact the pose suggests it was straining him.

Minato's complaints about Sage Mode are similar to how Naruto first used it (e.g. not holding it long, being slow, etc) suggesting that he only recently mastered it. It took him 3 years to create Rasengan, and it took Naruto the equivalent of years to learn Rasenshuriken (a week of training with 100+ clones is years of actual -constant- training).

So how far was Minato with the training? given what Naruto used Sage Mode for, it can be implied that Minato was running into problems with Rasenshuriken himself (since failure of the jutsu has quite a backlash). Combined with Hokage as a full-time job, Minato would likely have needed a few more years.