It sounds as though the printer has somehow been set for bi-directional printing, or the other way round. Look through the options of your printer software to see if there is an option to change this.
It may be that the printer itself does not support two way printing so it is printing only when the head is going from right to left but not printing on the next line down as the head travels from left to right.
This is only a guess but all I can think of at the moment. At least this will bring you back to the top for others to offer suggestions.