This issue is most likely configuration related. Check the mouse button and program settings in Logitech Mouse and Keyboard software (SetPoint) as described below:

NOTE: SetPoint will show your MK120 as Optical Mouse.

1. Launch SetPoint (Start > Programs > Logitech > Mouse and Keyboard > Mouse and Keyboard Settings).

2. Click My Mouse tab at the top and choose Optical Mouse from the Select Mouse drop-down menu (shown below):

   

3. Choose the mouse button you wish to customize in the Select Button field:

   

4. Choose the task you wish to assign in the Select Task field:

   

5. Click Apply to save your settings. Click OK to exit SetPoint.

Additional Information:

If the above solutions do not resolve your problem, consult Knowledge Base article 24916 for instructions on contacting Logitech Technical Support.