Here's why:
* The "a" in "complain" is unstressed. It's the second syllable, and the stress falls on the first syllable ("com").
* Unstressed vowels in English often reduce to the schwa sound. This is represented by the symbol ə.
So, the pronunciation of "complain" is kəmˈpleɪn, with the "a" sounding like the "a" in "about".