Product weights

I have just finished applying all shipping rules to all countries that we will ship to once the website is complete. I have used the weight/country/region system and have added their respective weight band s and prices. However, my products wont allow me to add a weight to them, rendering all of this work useless. I have noticed in the quick edit option in the products you have the option to add weights, but each time I do so and click apply, when i refresh it comes back to N/A.
please help