Thanks friends and fellow Power BI users for comments on this subject.
I've found the root causes now after some trials and errors.
Since I downloaded my data from an ERP system into excel for analysis, data types in excel do not always match the data type in ERP. What I've done is not to change data type in Excel, just do ETL process in Power BI Service and change data types there when loaded into service and later in Desktop when we have to edit querry and create data model.
Thanks very much