If the problem is due to a bug in the program code, there is probably nothing you can do except contact the developer and wait for them to issue a bug-fixed update.

However, just as with Macintosh applications, the problem may be due to a secondary cause, such as a corrupted preferences (.plist) file. This is especially likely if the symptom did not occur when you first used the app. If so, the solution is to delete the app's .plist file. If you have a jailbroken iPhone, you can again use iFile to navigate to the relevant Library/Preferences folder and delete the app's .plist file.

Otherwise, your best bet is delete the app from your iPhone and reinstall it. To do so, start by pressing down on the icons in the Home screen until the x's appear and then select the x for the relevant app. When you do this, a message pops up warning you that this action "will also delete all of the app's data." Go ahead; this not only deletes the app but should also delete its .plist file(s) as part of the "app's data." Now reinstall the app (you can either download a fresh copy from the iTunes Store or use the existing copy in your Mac's iTunes Library). Via any of these variations, you should wind up with a newly created default .plist file--hopefully with the symptom gone.