on the level America, the last part were you face the shield generator the portal is closed. I did find all the artifacts and i saved at the different points first went through the level and destroyed the processors then saved . After stage 4 i got the UFO in the hanger and went back to get the artifacts and crystals. i went into the crashed UFO in stage2 and flew that around and saved after that. I went to the portal but it was closed and it was opened after I beat stage4 but after that it was closed and stage4 area on the map was darked. so what is wrong here?