shipping is sometimes not being charged

I’m having a really strange issue, I have 2 shipping zones set up, one for continental US and one for Alaska/Hawaii, I have both of these set up as table rates with free shipping over $75, when it is working correctly i see a line item on the customers order that shows either $10 for shipping or $0 for shipping. The problem is that there are a number of orders (both above and below $75) that show nothing for shipping, they just don’t have a line item for the shipping charge, they are not charged $0, there is just no shipping at all. I have gone through all the documentation for shipping zones and table rates, I have tried recreating those orders to see if its a postal zone issue, i have tried changing from table rates to just flat rates, this is still happening with almost 1/5 of the orders and it is a really big problem. Any help is appreciated.