To Fivejedjon,
Actually, the vowel in 'on' is rarely weakened to /ə/ in any position. The only prepositions with common weak forms in BrE are at, by, for, from, of and to
Why do some preposition have weak / reduced form, such as '' at, from '' , while others don't have, such as '' on, up '' ?
Why ''on and up '' is not reduced / don't have reduced/ weak form ?
many thanks