Report not returning any data via API

I’m trying to download the GET_FBA_FULFILLMENT_MONTHLY_INVENTORY_DATA report using the API.
I’ve previously downloaded the end of month data using a start date of the 1st of the month and an end date of the 2nd of the month and it’s worked fine, but this month it keeps returning DONE_NO_DATA
I’ve tried various other date ranges and I either get DONE_NO_DATA or I get both the March and April end of month figures together.
2019-04-01 - 2019-04-02 - No Data
2019-04-01 - 2019-06-01 - No Data
2019-03-31 - 2019-04-30 - March and April figures
2019-04-01 - 2019-05-01 - No Data
2019-03-31 - 2019-04-01 - March and April figures

Am I doing something wrong or is there a problem with the data?
Example api call
POST
mws.amazonservices.co.uk
/Reports/2009-01-01
AWSAccessKeyId=xxxxxxxxxxxxxxxxxxxx&Action=RequestReport&EndDate=2019-06-01T00%3A00%3A00.000Z&ReportType=GET_FBA_FULFILLMENT_MONTHLY_INVENTORY_DATA&SellerId=xxxxxxxxxxxxxxxx&SignatureMethod=HmacSHA256&SignatureVersion=2&StartDate=2019-04-01T00%3A00%3A00.000Z&Timestamp=2019-05-07T10%3A49%3A21Z&Version=2009-01-01