Power Flow Using Matpower

Regarding the case57, the voltage of bus 31 is 0.936 p.u which is less than the min. limit of bus voltage (0.94). Every time I run 'runpf', the voltage value is 0.936 and it doesn't change although it is a PQ bus.

Can anyone help?

clear
clc
define_constants
mpc=loadcase('case57');
results=runpf(mpc);

[Walter Roberson]

(MATPOWER is http://www.pserc.cornell.edu/matpower/)