That's where my problem comes from. I'm not experienced enough to know immediately where failure is acceptable or not; rather, I don't always have backup plans or ideas for when things that should be able to fail, fail. So I roll, and it fails, and it should fail, but I've got no idea what happens when it does. So it doesn't fail.
I think I'm getting better at improv-ing events and making backup plans. It's still difficult for me to find the balance between the story I want to tell/ have prepared vs the story that my players wind up creating, but checking in with my party here and there tells me everyone's having fun and only rarely does anyone feel gipped or abused by dice rolls.