Have no idea as to the wiper kit fitted to your Morgan though have messed about with wipers on older cars over the years.
I might start by lifting the wipers clear of the screen and checking for free play on each of the wiper arms by rocking it left and right, if there is an issue of excess free play on any one arm compared to the others then it might be worth removing the arm on the suspect arm and checking the splines in it and on the driving wheel, I have seen these stripped to some degree.
The arms can over time size on to the driving spindle, and can be difficult to remove. There tends to be a small spring clip arrangement holding the arm to base of the drive wheel that requires to be prised and held back to allow the arm to be released.
The wheel boxes can suffer worn driven teeth where they run on the rack drive wire , dependant on the degree of arc there may be enough good teeth to disengage the wheel from the drive and rotate the wheel to present the non worn teeth to the drive.... Also if the drive wire has suffered wear, it too could be disengaged and rotated to present the unworn section to the teeth of the driven wheel in the wheel box..
Hope I have not muddied the water.
