Apple's Mail app, and other apps with an "Open in..." feature, will show up to a limit of ten items in the "Open with..." list if you are using iOS5 or earlier. All apps capable of opening the filename supplied, will be held in a list - but only the first ten are shown. The items are recorded in historical order. Apple suggest that only the top ten apps that the user will use for the file should be kept in the list. To remove an app from the list you need to delete it and, if the app is required, reinstall it. The app will then be added to the bottom of the list and may not be visible in the "Open with..." top ten if the number of appropriate apps exceeds ten. If currently listed apps are removed until the app is shown in the list, you will be able to call the app from Mail.
The problem is an Apple restriction, not an app issue. This was removed in iOS6 allowing more apps to be displayed.