Why does the behavour of my script change when it is built?

The script consists of a start and an OnGUI function. The purpose of the script is to display images from the resources folder and display them. The OnGUI lets you view the next page in the folder when a button is pressed.

This works fine in the unity editor but it does not when i build it. When it is build then the following happens:

First time i press the button, it changes the images and shows the next ones from the folder as it should. Second time i press the button it does not display the next images from the folder. Does anyone have an idea why it is working in the unity editor and not when it is build?

I use the Resources.load function to load the images as textured2 objects.