A: Apps may be listed in the App Store as iPad Only, iPhone Only, or they may show up in both categories. If you search iPad apps, but the app you are looking for is listed in the App Store as iPhone Only (even though it can be used on both devices) it will not show up in the results of the search.
An example is the TonePrint app from TC Electronic. If you search on your iPad, and have iPad Only apps selected, then it will not appear (even though it runs fine on iPads). To find it, set the search to iPhone Only at the top left of the App Store screen.