My iphone 4 won't let me download app

I had instagram before, and I accidentally deleted it the other day, now i'm trying to redownload & it won't let me. It says open next to the app, and even when I try to open it, it won't. I've restarted my phone, I've connected it to my computer and everything. What else is there to do?

Have you got enough memory left? Sometimes when it doesn't let you download an app its cause off that

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