Freight Charges

[JMCNALLY]

For some reason, occasionally the freight charges we have entered on a Sales Order will not be transferred over when we Invoice it. Luckily, we seem to catch it most of the time and enter it manually -- but it used to do this automatically. There is no rhyme nor reason as to when freight carries over and when it doesn't. Does anyone have any idea why this might be happening?

Thank you!