English has optional schwa deletion in words such as desperate, different, family, interesting and memory. In these words the optional schwa occurs immediately after a stressed vowel. However, English also has words in which schwa deletion is obligatory. This is indicated by the spelling.
Here is a list which compares words with and without a schwa. The words on the right exhibit schwa deletion:
In nine of the words with schwa deletion the preceding syllable is stressed. The exception is carpentry. Here the first syllable of the word is stressed rather than the syllable immediately preceding the one with the deleted schwa. The words on the right have suffixes attached to the root. The roots and the derivations have the same stress with the exception of two word pairs: ancestor/ancestral and theatre/theatrical. In these pairs the root has first syllable stress and the derivation has second syllable stress.
Many English words have a schwa which can be deleted. However, words such as actress and entrance don't have the schwa which is preserved in the root. This schwa was deleted and is evidence of language change.