Time Intelligence not working for monthly data

Hi,

A relative newbie and trying to create my first Power BI database and dashboard, working with monthly frequency data. To utilise the time intelligence feature, I have assigned a first day of the month date to each month and incorporated this in my date table (Power BI calendar) and the sales table (Master Data), however, the DAX functions for YTD vs PYTD do not work and seem to bring up the same result for Total Sales TY and Total Sales LY. Any ideas?

Total Sales = sum('Master Data'[Sales Value (£)])

Total Sales TY = TOTALYTD('Master Data'[Total Sales],'Master Data'[Date])

Total Sales LY = CALCULATE([Total Sales TY],SAMEPERIODLASTYEAR('Power BI Calendar'[Date]))