Sadly with pines, they are slow to show problems, and they are slow to recover. Sometimes by the time you see they are suffering, the tree is already under a great deal of distress.
If it were my tree, I would remove it from the pot as gently as possible and inspect the roots. If the tree is extremely root-bound, it may be difficult to get it out of the pot, but that will tell you right away that you have a problem
I don't know your experience level, but the tree may be wired into the pot as well, so check the drain holes on the pot to see if the tree is wired - and cut the wires in order to get the tree out.
A lot depends on the condition of the roots. If the roots look good and the soil is good, you can return the tree to the pot and we can go with "plan B".
If the tree looks severely root bound or you can see dead and rotting roots, I would then try to open up the root ball without cutting any roots, and check to see whether the roots are getting enough drainage and oxygen. Then I would gently transplant the tree into another pot/container that is about twice the size of the current one, in a good bonsai soil mix. This wouldn't be a permanent repot, rather an emergency "hospital" situation where you are trying to get the pine to recover and strengthen. Then I would leave it alone for a year and hopefully it will recover.