The high beams and low beams are two seperate circuits. The headlight switch feeds the dimmer switch and the dimmer switch chooses one or the other.
BUT the low beam bulbs have both a high beam and low beam circuit with two filiments in each low beam bulb, so all 4 light are on high beam, and only two on low beam.
Do the low beams not light on high beam, or do they not light on low beam, or not light at all.
If the problem occours on high beam you have power from the dimmer switch to the lights. Now a bad connection could be the cause. This could be on the power or ground side of the light.
If the low beams work on low, but not on high or the other way around, the ground is good, or they would not work either way, so look for a power side bad connection.
If the low beams work on high, but not on low, again it could be power or ground connections, or no power from the dimmer switch.
If they don't work on either, ground is more likely the problem.
Don't forget, you could just have bad headlight bulbs. Yes they can flicker with a broken filiment when bounced around.
Ken Styer