App's Install button missing on Iphone's apps store screen

App's Install icon missing on Iphone screen. After updating to ios4 when logged on Apps store unable to find install icon/button on the info page of an app. Use to work ok. Using itunes on the pc is ok apps download and install. I have done a phone reset as advised by the apple store but still no good. The chaps at the Apple Store had never seen the problem before! Some one also said that there was a setting to allow app down load but I can't find it any where on the phone or on the pc/itunes .All software has been updated ....help please