Você tem linhas muito longas (largas)? Você viu line-number-display-limit-width ?
line-number-display-limit-width is a variable defined in 'C source code'. Its value is 200
Documentation:
*Maximum line width (in characters) for line number display.
If the average length of the lines near point is bigger than this, then the
line number may be omitted from the mode line.