dificulties to select brackets or braces

Hello all,
My problem souldn't be serious for specialists but rather annoying for me.
I am a beginner with Notation Composer and I don't master the selection tool (big arrow).
When I want to delete braces or brackets I can't select them at once. (they should become blue). I have to try several times and 1 out of 20-30 trials it works but I don(t know why......
Surely there is simple way to do that.
Thank you for your advices.