...do you have a logo inserted into the header section? It could be that your printer is out of memory when printing that specific document and thus prints blank instead of the logo. Perhaps you have to send few invoices at a time to the printer, to see if that helps.
Sometimes this occurs if a farily large logo is reduced in size within the Softrak form designer. Although it prints in a smaller format, it contains he same number of pixels and takes the same amount of printer memory. Often you can reduce the real size of the graphics file with a drawing program, even MS Paint, and save a smaller version of the logo for use in Adagio forms.