IPhone app trouble not downloading right?

Help I downloaded an app recently and just about when it gets to the end it says the app was unable to be downloaded. If I try to open it it automatically closes. I'm downloading it over 3G but its under 20 mb so it should work fine I've tried turning it off and on and resetting the app but nothing works what is wrong

It's probably an app that has a lot of bugs! Many of them crash and immediately go to the home screen after you've tapped it.