Not enough bugs?

Coverage can sometimes be improved by specifying a dictonary
(-x dictionary-dir). Some dictonaries are included with afl.
For example: /usr/local/share/afl/_extras/png/

"Somewhat predictably, when the syntax tokens are around
1-2 bytes long (as is the case for GIF and JPEG), the benefits
of fuzzing with a dictionary are fairly modest and the ultimate
coverage does not differ much. For data formats that rely on
longer atomically checked tokens (e.g., 4-byte PNG section
headers), the gains are are much more profound."
/usr/local/share/afl/testcases_readme.txt