Surely, Malbim expounds "va'Yimalei Shiv'as Yamim Acharei Hakos Hashem Es ha'Ye'or" (Shemos 7:25) - the seven days that were allotted from the beginning. Perhaps Malbim explains that Pharaoh did not know the limit, and by chance, on day seven he asked Moshe to remove it. The Torah omits his request, for in any case the Makah would have ceased that day. (PF)

Moshav Zekenim (Shemos 8:14): Arov and Dever came by themselves, not through the staff, so Pharaoh suspected that it is chance. When Arov left through Moshe's Tefilah, Pharaoh knew that it was from Hashem. Dever did not leave. (However, Malbim says that Pharaoh attributed also boils to chance, even though Moshe and Aharon initiated it in front of him (Shemos 9:10)! - PF) Malbim (Shemos 9:7) - one animal died, of a man born to a Yisraelis from an Egyptian. Pharaoh mistakenly thought that he was a Yisrael, so he did not attribute the Makah to Hashem.

Malbim discusses why Pharaoh did not ask to remove Dever. This verse was said between boils and hail! This requires investigation. (PF) Ramban says that he asked to remove only Makos from which people were dying; R. Yonah says that he asked only about Makos that kept getting worse. In Dever, all Egypt's animals already died!

Why is this a reason not to ask to remove it? Perhaps it is because it is a punishment for the past, and not due to current refusal to let Yisrael go. (PF)