On my 300 disco if I put the headlights on there isn't a problem but if I try to pull the stalk back for main beam the headlights switch back to sidelight

Give the stalk another pull and the headlights come back on , when I pull the stalk back once I can see everything light up inc my 2 spots but if I try with my headlights on it won't keep full beam . Is it the stalk or something else ? Cheers