Arm will not stay attached to window

Thanks for writing in! This casement operator reminds me of our 39-329 LH Inverted Split Arm ROTO Operator. The arm connects to the bottom of the window by attaching to the bracket at the bottom corner of the window. There is a clip on the bracket that snaps onto the stud at the end of the arm. You'll need to examine your bracket to see if the clip is damaged in any way. If so, you'll need to replace yours. 

Your bracket appears to be the 39-553.

I hope this helps!

Please let me know if you have any questions.