The problem is, for example when creating a new table from import, and having fields with 0s and 1s, it imports. then you change the field type to checkbox, and everything works.
Now, if you already have those fields as checkbox, and delete all the data from the table for ex, then import back in, the fields doesn't recognize the data and every checkbox field is just blank. the system should already determine when importing back the data to map correctly to the checkbox type field. and dont omite data.
Best regards, Lorenzo