The schwa sound is commonly used in unstressed syllables, while the full vowel sound is typically used in stressed syllables. In the word "again", the first syllable is stressed, so it is typically pronounced with the full vowel sound. The second syllable is unstressed, so it is typically pronounced with the schwa sound.
However, it is also possible to pronounce the word "again" with the full vowel sound in both syllables. This pronunciation is more common in certain dialects and accents.