Why, on some occasions does Moshe warn Pharaoh, and on others, he does not?
Ramban: Refer to 8:12:2:1.
In similar vein to the Halachah that a person requires warning before receiving Malkos the first and second time he transgresses, but not the third time. Because the ten plagues came in three independent groups, as the Si'man 'D'tzach, Adash be'Ach'av' indicates - representing three on the ground, three above the ground and three in the sky; or, as the Seforno explains the first group took effect on the two elements water and earth. The second group on living creatures and the third group, on the air. Makas Bechoros is not really part of the third group (Be'ach'av - since, unlike all the others, it came, not as a warning to Pharaoh to relent, but as a punishment for failing to do so - Ramban), and is only mentioned together with it for convenience.