A.T. Robertson Commentary Luke 20:26

"And they were not able to take hold of the saying before the people: and they marvelled at his answer, and held their peace." — Luke 20:26 (ASV)

They were not able (ουκ ισχυσαν). They did not have strength. An old verb ισχυω from ισχυς (strength). They failed "to take hold (cf. verse 20) of the saying before the people." These "crack" students had made an ignominious failure and were not able to make a case for the surrender of Jesus to Pilate. He had slipped through their net with the utmost ease.

Held their peace (εσιγησαν). Ingressive aorist active of σιγαω. They became silent as they went back with the "dry grins."