Sumx formula giving error for one record

Dear Users,

I have been racking my brains over this issue since yesterday. any help will be greatly appreciated.

I have two tables in Power BI, Case Table and Paid Table. both are joined by a unique reference number highlighted in Green.  

In the Case Table, I have used a formula in Column G

Payment = SUMx(Paid, if(Paid[Ref-Event Month]=Case[Ref-Event Month], CALCULATE(SUM(Paid[Payment])),BLANK()))

I have over 7000 records, and all of them are correctly displaying the payment amount with the exception of one reference. 

I have checked over and over in the Paid Table, but all the amounts add up to £204, whereas the Case Table is showing £244 for this reference.