At technical level, fixed bin will be determined based on the below logic : system will first check for the fixed bin field at the storage type level , if this is checked ( /scwm/t331 - fixbin = 'X' then system will call FM ' /SCWM/BINMAT_READ_MULTI' which will read the data maintained through the bins maintained in the /SCMW/BINMAT transaction at the front end level to fetch the bin data.
At times, we will struggle with stock being not picked from the source bin, in this situation to investigate from technical level for issue related to the source bin determination, try to keep breakpoint on the FM ' /SCWM/REM_BIN_DET_INT' which will give you some hint related to the root cause of the issue