That depends on how you have your db setup. If you have the columns in this table set to NOT NULL then you will not be able to leave them blank when inserting or updating a record. If you would like to be able to have empty values for these columns you will need to alter your table and take away the NOT NULL attribute for these columns.
If this is not the case for you and you are not using NOT NULL columns then the issue may be somewhere else within the query or the code. If you can post back with a copy of your update I should be able to take a look to see what I can spot. Can you insert blank values on the insert page?