Automatic payment to vender through APP

Hi Experts,

I am facing problem in APP. I created APP and document get posted to vendor a/c. But when I see in FBL1n it is showing open item both invoice and payment doucment(ZP). I tried it to clear through auto. clearing functionality. but it didn't. what could be the reason? and how to clear open item?

Aprreciate ur prompt response.

Thanks in advance