וַיַּכְבֵּ֤ד פַּרְעֹה֙ אֶת־לִבּ֔וֹ גַּ֖ם בַּפַּ֣עַם הַזֹּ֑את וְלֹ֥א שִׁלַּ֖ח אֶת־הָעָֽם׃ (פ)
И на этот раз фараон ожесточил свое сердце, и он не отпустил людей.
Rashi
Again this time. [Once more,] though he said, “I will send you out,” he did not keep his promise.
Ibn Ezra
And Pharaoh hardened. He hardened his heart like he did with the frogs, it is written there "and his heart hardened"
Rashbam
גם בפעם הזאת, just as G’d had stiffened Pharaoh’s heart after the relief experienced when the frogs had died, so that he had reneged on his promise.