If you bench tested the bulb or switched them to test. And your right low beam is working properly you need to check 2 things.
First grab a 12 volt test light and turn on lights. making sure the tester is properly grounded then put on high beams and check the plug on the car. there should be 3 prong hole. 1 of the holes should make the test light come on. if it does the turn on low beams and check the other 2 prongs. If your test light comes on then you have a bad ground. if not you have either a bad switch or bad wire. probably a bad wire. Check the wire's as far as you can. looking for an exposed spot or break in the wire. if you find nothing the u should change the headlight switch. If this doesn't help your last resort is to the it to an automotive electrical shop to find the break in the wire between the switch, firewall or plug.
85% of the time this fails because of the ground.