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.
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Launch SetPoint (Start > Programs > Logitech > Mouse and Keyboard > Mouse and Keyboard Settings).
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Click My Mouse tab at the top and choose Optical Mouse from the Select Mouse drop-down menu (shown below):

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

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

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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.
Important Notice: The Firmware Update Tool is no longer supported or maintained by Logitech. We highly recommend using Logi Options+ for your supported Logitech devices. We are here to assist you during this transition.
Important Notice: The Logitech Preference Manager is no longer supported or maintained by Logitech. We highly recommend using Logi Options+ for your supported Logitech devices. We are here to assist you during this transition.
Important Notice: The Logitech Control Center is no longer supported or maintained by Logitech. We highly recommend using Logi Options+ for your supported Logitech devices. We are here to assist you during this transition.
Important Notice: The Logitech Connection Utility is no longer supported or maintained by Logitech. We highly recommend using Logi Options+ for your supported Logitech devices. We are here to assist you during this transition.
Important Notice: The Unifying Software is no longer supported or maintained by Logitech. We highly recommend using Logi Options+ for your supported Logitech devices. We are here to assist you during this transition.
Important Notice: The SetPoint software is no longer supported or maintained by Logitech. We highly recommend using Logi Options+ for your supported Logitech devices. We are here to assist you during this transition.
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