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Do you get any error (for example 902, 502 or so) when trying to download the app? The easiest way to fix Market errors is to delete Market cache or Market data. Just go to Settings / Applications select "All", find Market (or Store Google Play), choose it and select "Delete cache", then try if Market works. If this doesn't help, select "Delete data" on the same settings screen (don't be afraid, it will just delete Market data, no applications) and this should fix it. If no, try to restart the phone.