Dying swan act of my Pinus wallichiana (Bhutan pine)

Can anybody advise me why my Pinus wallichiana (Bhutan pine) may be dying.
I planted two of these in the same soil about three months ago. Both are getting sun and the ground is a mix of damp clay and some sand mixed in.
One seems to be doing ok and the other has turned brown and shed most of its needles.
I'm a very novice gardner. Both have been kept watered when needed and not over watered.

Click image to enlarge