Data Entity is missing in Import Data Management

I have made an extension of SalesOrderLineEntity and added a customized field from Salesline Datasource to the fields of the entity.

After rebuild, i have tried to use it to Import in Data Management, but the data entity is not showing in the Entity Name drop down in the Import.
It is showing before i have made the modification in the extension. 

I have tried to Generate mapping in source, also full Build, but still not showing.

I have also tried to just type it and not go with dropdown and it is working. but not showing in dropdown.

Any help with this issue, thanks.