Wishlist member class not found in wp-admin

Hi, I am creating a wordpress plugin that adds an action to a wishlist member plugin hook. In the function, I need to use the WLMAPI class of wishlist member. The problem is the class is not found.

I tried a test code in using the class in functions.php and it was found. Also I create another test folder in the root folder and create a test script to call the class and the class is found using class_exists function.

I don’t understand why the class is not found in the plugin that I created which resides in the plugin folder. Any ideas?

Thank you!