Dear Yossi,
Pls check C223 for number of BOM's Exists based on Plant and Material...
Then Check CS03 for BOM Display ..
In CS03 : Enter Material, Plant and BOM Usage (By Default BOM Usage : 1 for Prod BOM).
It will Show the List of BOM's with Alt BOM .
Pl refer the below link for BOM Help :
http://help.sap.com/saphelp_46c/helpdata/en/ea/e9af734c7211d189520000e829fbbd/frameset.htm
Rgds :
Lokesh