You need to check the flow and see if Lead Score is also being decreased. They have to be in the same flow for this to work.
Ok, more clearly:
Trigger = X
Flow =
Behavior Score -10
Lead Score -10
Otherwise Lead Score is NOT decreased automatically. You must always do this on ANY scoring flow. Also, if you ever reset Behavior Score, be sure to also do this:
Change Data Value: Lead Score IS {{lead.Demographic Score}}
so that the Lead Score is properly decreased.