Timestep UDF not work
 

Hi,all
I have met a problem in timestep UDF.The following is my UDF code.
DEFINE_DELTAT(mytimestep,d)
{
real time_step;
real flow_time = CURRENT_TIME;

if (flow_time < 0.005)
time_step = 0.00005;
else if(flow_time < 0.025)
time_step = 0.0001;
else if(flow_time<0.175)
time_step=0.0005;
else if(flow_time<1.175)
time_step=0.001;
else if(flow_time<5.675)
time_step=0.003;
else if(flow_time<10.675)
time_step=0.005;
else
time_step=0.01;
return time_step;
}
After i hook my UDF to Fluent and start up the iteration,but only one time step can be run, then iteration was stopped and can not work continue.Who can help me to find out the reason.Thanks.