am trying to import a mid-life assets so i updated the assets template with the required data as mentioned in SA and after that i exported the Internal ID and update the assets record with the current net book value then i run the script to "Precompute depreciation value" and it sometime works and other not

Just apply first principles. Identify what is common in the setup of the assets that don’t work the way you expect. Then you have data with which to resolve your issue.