A.T. Robertson Commentary Romans 9:32

"Wherefore? Because [they sought it] not by faith, but as it were by works. They stumbled at the stone of stumbling;" — Romans 9:32 (ASV)

We must supply the omitted verb εδιωξα (pursued) from verse 31. That explains the rest.

They stumbled at the stone of stumbling (προσεκοψαν τω λιθω του προσκομματος). The quotation is from Isa 8:14. Προσκοπτω means to cut (κοπτω) against (προς) as in Mt 4:6; John 11:9f. The Jews found Christ a σκανδαλον (1 Corinthians 1:23).