Hi,
You are running into bug 25384. In Flexible Billings, Invoice and Adjustment Maintenance, the Rate is zeroed out when tabbing down to the next line. However, after saving the line, the Rate reappears. This only happens when you have no transactions loaded into the detail area and you haven't yet saved the draft. After once saving the draft, the lines then retain the rate.
There is no fix for this at the current time other than to save the draft so that the rate reappears.
I'm sorry for inconvenience. Let me know if the above description matches the issue you are reporting.
Thanks.