French dressing will not blend or mix to be creamy I make a french dressing >  I blend using a milk shake mixer. Usually as I add the oil, sugar and vinegar it mixes up to a very nice creamy thick dressing. Lately it will not thicken up and stays very liquid almost like a vinegar and oil.   

Is there a sequence to mix the oil, vinegar, salt and sugar?
Try adding a bit of egg yolk. The lecithin in that usually helps these things emulsify. Obviously you can't do that if you're young/pregnant/old.

If you don't want to try egg yolk. Put everything in but the oil. Add maybe half the oil and shake, then add a bit more and shake. Do it piece by piece as if you were drizzling into a bowl and mixing by hand with a whisk.