Cannot get quarterly difference matrix to work

I am trying to build a matrix that compares the differences from one quarter to another - actual differences or percentage differences.

I have tried copying several examples without success. The examples use numeric values (e.g. Sales in USD), but the values I am using are not numeric, but strings (e.g. "Sales visit" or "Contract signed"). Therefore I cannot use SUM and have been trying with COUNT instead:

SupportTypeCount = COUNT(MyTable[Support type])

In the matrix the rows are the "Support types" and the columns are Quarters:

Quarter = FORMAT([Date], "\QQ yyyy")

So far, so good. But I cannot find the correct measure for the values - I have tried numerous combinations, also using a Date Calendar, but all have failed:

QuarterlySupportChange = DIVIDE(MyTable[SupportTypeCount], CALCULATE([SupportTypeCount], DATEADD('Date'[Date], -1,QUARTER)),0)-1
Supports LQ = CALCULATE([SupportTypeCount],DATEADD(MyTable[Date],-1,QUARTER))
Supports LQ = CALCULATE(COUNT(MyTable[Support type]),DATEADD('Date'[Date]-1,QUARTER))
Supports LQ = CALCULATE(COUNT(MyTable[Support type]),PREVIOUSQUARTER(MyTable[Date]))
Supports LQ = CALCULATE(COUNT(MyTable[Support type]),DATEADD(MyTable[Date],-1,QUARTER))

Any help pointing out my error will be very gratefully received!

CJ