Sometimes this is also happen in my applications in my iPhone. Check the application if they have a new version or updates. Then update the said application and try it. Having a low or full memory can also the problem. Delete some files from your iPhone and try loading again the application.
You might also have a lot of open apps. Double click the centre button to bring up the tray with all open apps. Close all of them. Also try switching off and on the iPhone. Of course, if ur iPhone is an older version it mat be slower to load many if the newer apps.