If MATLAB thinks the figure is too complicated it will produce an embedded image in the output file, rather than vectorized (editable) content. You can force MATLAB to produce vectorized output by using the -painters option when calling the print command, as in:
print -painters -dpsc output.ps
or
print -painters -dpdf output.pdf
The output files will then contain vectorized content instead of embedded images.
See the Vector Graphics File discussion in the description of the formattype input argument in the documentation of the print command for more details.
