DAX can't find columns for dates

Hi,

I've been successfully using these sorts of DAX formulas for current month or week:

Current Month = if(MONTH(NOW())=[MonthOfYear],"yes","no")

IsCurrentMonth = IF (YEAR ( 'Invoked Function'[Date] ) = YEAR ( TODAY () )
&& MONTH ( 'Invoked Function'[Date] ) = MONTH ( TODAY () ),
"Yes",
"No"
)

Yesterday I found that it can't see the fields I want it to in the tables.  I get messages like this:



I've not done anything differently from a report I made earlier this week, so I'm confused why it can't seem to see the columns anymore.  The only difference may have been I updated to the latest version of the desktop.  Anyone experienced this before?

Many thanks