apps downloaded to iPhone 6 greyed out

I have an iPhone 6 running iOS 11. When I try to download the eBay app from the App Store, the circle showing the state of the download goes round to 3 quarter complete and then stops and goes back to the initial state. The app is on the Home Screen but greyed out and cannot be opened. I have tried all the obvious things, delete app from screen and iPhone storage, reboot phone but it always does the same behaviour. Any ideas as to why this is.?