I have no clue how you are whiffing 5c over the opponent's head. Like Indigo said, just do the jc right immediately after the j2a, no delay needed. As for me, I always super jump after every followup off of a 2c. For example, 2c-superjumpj2a-AD-J2A-JC.
Why don't you look at videos online so you can visually see the correct timing for these combos?
A very standard corner combo would be:
136b-2c-sj2a-j2c-2c-j2a-AD-j2a-jc-5c-3c. Once you master this, (which shouldn't take long), try to add in a 3rd loop of 2c-j2a-j2c, so:
136b-2c-sj2a-j2c-2c-sj2a-2c-sj2a-j2c-AD-j2a-jc-5c-3c
In order to do add the 3rd loop, you need to make sure the falling j2c's connect *right* before the opponent hits the ground, and you have to do super jump 2a's off of the 2c's.
Again, watching combo vids will help immensely