While inside the Work Incident form if you click on the 'Extend Classification' button no action is taken.
The Classification label's size definition to the left of the 'Extend Classification' button extend over the top of the button. Thus when the page is rendered the browser believes the button is behind another form component & is no longer clickable.
Symantec development has a fix available to address this issue.
TECH231874 contains the fix for this issue and all point fixes for ServiceDesk 7.6.