why feed importer cron fails?
I got the answer to this problem.

I used debug code if ($hook == 'cron') watchdog('cron', "hit $module cron"); in function module_invoke_all()

where to put debug code ?

In module.inc file of core drupal, add

 $function = $module .'_'. $hook;
 $result = call_user_func_array($function, $args);


between this two line of module_invoke_all() function, add above debug line.

It will look like :

$function = $module .'_'. $hook;

if ($hook == 'cron') watchdog('cron', "hit $module cron"); 

$result = call_user_func_array($function, $args);


After putting above line in module.inc,run cron manually and check log in recent log entries.

Because of this debug statement, I found the module which was not allowing to run cron.

I corrected the custom module code in that and cron started working fine.