Step 1: first GRN is done for the material and plant i.e movement code 101.
Step 2: after the zprogram schedule in background job every 5 mins, try to do post the same quantity for movement code 301 using the BAPI_GOODSMVT_CREATE,
Step 3, sometimes full quantity is not posted . we are passed full quantity of GRN to 301 posting in FM.
Step 4: Step 3 is occurred only 5 or 10 times per month. other 101 list are posted with full quantity for 301 movement code.
Any functionality behind partial quantity for 301 movement. we can't track this. For same material it post full quantity and some times it post partial quantity.