nbound Idoc Status 64

Hi experts,

Im facing diffuculty to find out why some IDocs are in status 64 on the system. If it is re-processed individually, it will be successful. But as a bulk, the status will be in 64. I checked the settings too. It is in "trigger Idoc immediately" status only. When these Idocs are recieved , the initial status is 64. Then it is taking time to change the status to 53 , one by one.....which is a gradual process. can you please help me out.