my Alarm doen't start activity when phone lock is

my Alarm doen't start activity when phone lock is

I try to do alarm clock in my aplication, but when phone lock is there
activity doest't start as in standart alarm clock. What I can do for
decide this problem?
Calendar cal = Calendar.getInstance();
cal.add(Calendar.SECOND, 5);
Intent intent = new Intent(this, AlarmReceiverActivity.class);
PendingIntent pendingIntent = PendingIntent.getActivity(this,
12345, intent, PendingIntent.FLAG_CANCEL_CURRENT);
AlarmManager am =
(AlarmManager)getSystemService(Activity.ALARM_SERVICE);
am.set(AlarmManager.RTC_WAKEUP, cal.getTimeInMillis(),
pendingIntent);