Probelm changing down gearbox

when changing down the gearbox from 5th to 4th and 4th to 3rd, it baulks, cannot select the required gear, only does it when above 40mph from 5th to 4th, and above 30mph from 4th to 3rd, no use if wanting to overtake or when using engine braking, no prolem when changing 3rd to 2nd, Garage have lubricated the gear linkages, and changed the gearbox oil, is it the clutch or something wrong with the gearbox ??