Cube View Cash Flow Report QTD or MTD

I would guess that your base CF accounts are Balance account types rather than Flow account types. While changing them is an option, I can't suggest that it will fix your issue because I don't know your build.

A second option would be to use the Flow dimension to help. There's a property called Switch Type which can be set to True. It looks at the account type (in your case, Balance) and switches it to act like a Flow account type when using that Flow member.