It is normal. Using Vials to obtain Breakthroughs and Trials is based on chances or the so-called RNG. The more you lock the fewer chances of getting gold skills also. You're not the only one who experienced or experiencing this kind of luck, even me had this problem with my Kushina and just yesterday, with my Minato [Edo Tensei]. If you're trying to get your last skill then you might opt to take frags way than trying your luck and spending way much than the frags way. Good luck though, thank you.