How do I make a Leek foam from Agar or Egg Whites?
Agar is basically just a gelatin. Lecithin is a surfactant and emulsifier, which is what you ideally need for your foam. The agar probably isn't sufficient to create the necessary tensions between the oil and liquid and when you introduce air into the mixture it isn't being held in suspension, which is why your foam isn't 'holding up'.

I'd strongly recommend trying lecithin along with a more vigorous air injection method such as an electric hand blender.

I don't believe it's anything do with your leek jus, but your method of foaming.

There's a useful article on this here.