I would assemble the brakes without the spring, bolt the arm in firmly, and check if it can move freely. If not, as stated above, you need to make sure the post is clean and then, carfully keeping it perpendicular, shwve the end of the arm bushing, A second reason might be the large MA of a V-brake makes any friction in the cable more forceful. This needs to be countered by the arm spring. Looks like you have plenty of threads in the adjustment screw to add some spring tension.

I would assemble the brakes without the spring, bolt the arm in firmly, and check if it can move freely. If not, as stated above, you need to make sure the post is clean and then, carefully keeping it perpendicular, shave the end of the arm bushing, A second reason might be the large MA of a V-brake makes any friction in the cable more forceful. This needs to be countered by the arm spring. Looks like you have plenty of threads in the adjustment screw to add some spring tension.