Create a third octave spectrum from a time signal
Hate to answer this question, but it is a plotting issue, not the actual design problem. Basically, you don't have enough points at low frequencies cause they are spaced linearly and you are plotting on logarithmic scale. You could do:

[h,f] = freqz(b{nn}, a{nn}, logspace(log10(fMin), log10(fMax), 1000), fs);


Explicitly specifying the logarithmically spaced bins. Or you can use a very large N (something like $2^{16}$).