I just tried to upgrade my skill level for my 7* red dragon and used 6 - 5* Satyrs so I would have a 90% chance of getting the skill up and didn't get it. This was the first time I was able to acquire 6 - 5* Satyrs of a specific color and it is extremely frustrating since it takes a while to acquire that many Satyrs and it is virtually impossible to get 7* or 8* dragon to skill level 16.
My suggestion would be to make it a 100% chance of getting a skill level up if you are using 6 cards. Probably the most disappointing part of this game is failing to get a skill level up with a 90% chance.