Work order to sales order link

SUGGESTED

I'm having issues with linking existing work orders to existing sales orders.  I can link the sales order to the work order using work order maintenance but only for some sales orders.  For example I have a existing work order and I choose the correct customer and then try and go to select the sales order.  When I access the field the sales order appears but when I select it the system exists the selection field but does not put the data in the field.   However, I can select another sales order (old and closed) and it will enter the data.  It doesn't seem to be related to hold status as I have both failed and succeeded with orders on credit hold.  Any ideas??

Patrick 

PS the failure of the linking process seems to be tied to whether the Sales Order line has ANY change made to it.  Any change seems to keep the linking from working.