Why do I stutter with vocals (A,E,I,O,U)

Why do I stutter with vocals (A,E,I,O,U) Hello, I'm 15 years old I got a stutter since I'm a kid, when I try to say a difficult word, the word simply wont come out, I open my mouth but no sound is made, over the years I improved a lot, to the point I can get over a stutter easily. This is not the case for words that start with vocals, for example the word exam, ego, interrupt, ago, old. I speak both english and spanish and I stutter with both languages and as I told you, the words that I really struggle with, are those who start with the vocals. I would love to know why since I cant find anything on the internet, also some tips to get over this. Thank you and I hope all of you can get over this problem