This usually happens when the control knob cap is not fully locked during use and can be resolved by checking and securing the components below.

Causes and solutions

Check the control knob cap
Make sure the control knob cap is not loose. If it is, rotate it until it is fully locked to prevent it from coming off during adjustment.

Check the upper and lower end caps
Inspect the gap between the upper and lower end caps of the control module. 
If the gap is too large, press them together first, then insert the control knob cap and rotate it clockwise until it is fully secured.