When I submit the SAS program as below, the log only shows the line numbers read successfully but no output displays in the OUTPUT.  I clear all the log and output and repeat the program several times, the same happens.  However, when I quit and invoke SAS again, the program runs and output is shown.  This problem happens frequently in running the program.  What is the problem ?
e.g.
options PS=90 LS=120 nodate nocenter nonumber;
libname temp 'c:\work';
data tt;
infile 'c:\t.txt';
input year 1-4 district 5-14 residence 15-24 travel 25-27 weight 28-42;
run;
proc sql;
     create table temp.work as
            select year, district, travel, weight
            from tt where year=2009;
quit;
run;
data test;
       set temp.work;
       if district in (1,2,3) then travel=99;
run;
proc format;
     value mode 1='Train'
                2='Car'
                3='Plane'
                4='Ship'
                5='On foot'
               99='Others';
run;
proc tabulate data=test F=comma12. noseps;
     class year district travel;
     var weight;
     format travel mmode.;
     table district=''*travel='' all='Overall', year=''*weight=''*sum='' /rts=25;
run;
Please advise.