RunningCount not returning expected output

I'm trying to get the number of consecutive months w/o delivery (column 4) for attached dataset. As you can see, I want to reset the counter in case there is a delivery for particular month. I tried to use RunningCounter function as shown below.

=RunningCount([Delv_Qty] ; ([Ship_To]))

It is not resetting the counter as Ship to is not changing. I think I need additional variable to achieve this.

How can I achieve desired functionality?

Thanks,

Milind