Turning vanes added at 90 elows mear rhe furnace ,or where acessable.

Increase the size of the return grille(s0,or add additional return ducts and grilles.

Parallel/Helper trunk ducts for supply or return.An example would be the trunk going to the second floor attic is too small and it's encased in the walls.You find another place to run a Helper trunk from the basement to the second floor attic,tie the two trunks into a new larger trunk in the attic and the issue is solved,assuming you size the second one correctly.