Hi Nicky,
Just a quick description of how RUNALLOCATION works. It selects records based on your WHAT restrictions and then records based on your USING restrictions. Then it tries to find records from WHAT that match records from USING. Match means that members in "complementary" Dimensions are same. In your case, I can see that TIME is "complementary"; can't tell if you have more in the Model.
If, for example, all WHAT records have TIME = 2016.01 and all USING records have TIME = 2016.02 that's when MERGE is going to produce no records.
Hope you got the idea.
Regards,
Gersh