I'm a bit confused by these words. Since "nonsense" derives from "sense" but the vowel in "sense" is /ɛ/ and it reduces to /ə/ in "nonsense". BUT "nonsensical" derives from "nonsense" and the vowel in it is the same as "sense". Please explain.
This is because of weird stress patterns of English. The stress patterns of English are very complex.
Sense is a monosyllabic (one-syllable) word and is stressed, so the vowel is pronounced /ɛ/ and does not get reduced to a weak vowel (schwa).
However, when you prepend the prefix non to sense, the primary stress moves from the second syllable of 'nonsense (i.e. 'sense') to the first syllable 'non' because it's a disyllabic noun and disyllabic nouns are almost always stressed on the first syllable. Therefore, the vowel in the second syllable gets reduced to a schwa /ə/ (vowels in unstressed syllables often get reduced to schwa). Therefore, 'nonsense' is pronounced ['nɒnsəns].*
On the other hand, 'nonsensical' is pronounced [nɒn'sɛnsɪkl̩]. The vowel in the second syllable doesn't get reduced to /ə/ because it is stressed now. Like most adjectives that end with -ical, 'nonsensical' is stressed on the antepenult (third-last syllable). Therefore, the vowel in its second syllable doesn't get reduced to /ə/.
(*This case is more complicated but I'm not going to explain it further because it will confuse you.)