Bizarrely, in English, the most common vowel sound has no single glyph to represent that sound. Among the many variations of sounds that vowels can make, all of them can be pronounced as 'ə' (schwa) in certain circumstances. This is part of what makes English orthography a nightmare and then pronunciation of written words difficult.
As an example: "banana" has two different vowel sounds, 'ə' and 'æ' (in American English at least). If English allowed for 'ə' it could be spelled bənanə.
Other words with schwa:
amazing - əmazing
tenacious - tənacious
replicate - repləcate
percolate - percəlate
supply - səpply
Not all spoken Englishes might agree with all of these, but every English variant uses the sound.
