Changing the order of leaf member attributes

Hi Guys

So I have an entity that consists of all of the months of the year. I added new leaf members named Jan_Invoice_No, Feb_Invoice_No, Mar_Invoice_No etc. I would like to order the columns so that Jan_Invoice_No comes after the column named January. So each month must be followed by the Month_Invoice_No.

When I click the up arrow to move the new leaf member it sends it to the beginning of the list. When I click the down arrow it sends it all the way to the end. Normally it moves past one member at a time until it's where you want it.

What is causing this to occur?

Thanks