If you are publishing an item with a tax amount different than zero and it goes to Kashflow without tax, then you need to switch on your "Publish Tax Data" option.
This is done from the Account Settings menu under the Integration tab:
In that way, the tax amount extracted from the item (or preset by Supplier rules) will be published to Kashflow.
Please note that Dext Prepare will only send the tax amount but not the tax rate used for it.