Don't know why one is smaller but all would benefit from less grass around the base and more organic mulch. After you clear out the grass and apply 3-4 inches mulch, apply some high nitrogen fertilizer at regular intervals. The fertilizer needs to be ammonia based and designed for acid loving plants. Blueberries also need to be kept moist. They need soil with pH around 4.5. High pH is a common cause of failure.

A good idea would be to place the ailing plant in a pot. In the pot simply put a mix of good soil, peat moss and pine bark fines at a ratio of 2:1:1. Give the plant a watering every 3-4 days leaving soil damp but not soaking. Keep plant in full sun and look for improvement within 2 weeks. Signs of improvement would be new leaves coming in and healthy looking bark etched. Good luck!

Do you fertilize this late when they are clearly going dormant?

Silva-Moon looks to be in New Zealand,if that makes a difference. Brady