Drillthrough issues - doesn't work with date slicer

Hi all, 

I am currently experimenting with drill throughs, and can make them work mostly - but have an issue that I will try to explain. 

My dataset is simply: user, country, datearrived, dateleft set-up in a matrix - I have linked the date fields to the date table (one active, one inactive and using USERELATIONSHIP as appropriate for date logic sums). I then have a slicer for dates so I can quickly view arrivals/depatures in a month (my date table has a MM-YYYY column that I utilised for this slicer). 

However, when I set up the drill through in Power BI the country works in producing a drill through, but the date fields do not produce any filtering in the final version after the slicer has been applied - they will provide a top level amount as if the date slicer had not been used, but will not filter things down further than that. 

Is there a beginners guide to drillthroughs that I can look at to assist with the process and make sure I've ticked off everything I am supposed to have done?