Clean the machine.
Also check your ribbon color selector function for proper operation. When set to dark it should raise the vibrator X amount, when set to red, it should raise it half a ribbon-width more. What this switch also does is disengage the ribbon advance when set to the white position, so if you have it set to dark but the linkage is bent/loose/dirty/etc. you may have a condition where the ribbon vibrator will raise but the advance may think that the selector is set to the white position. Cleaning usually takes care of most problems, but the ribbon advance and vibrator linkages can be finicky on all machines. Good luck.
Phil Forrest