Make sure gulp displays scss/js error messages on the terminal when running. Example error:
[18:51:06] Starting 'styles'...
wp-content/themes/timber/assets/scss/components/_hero.scss
Error: Invalid CSS after " color": expected "}", was ": $white;"
on line 8 of wp-content/themes/timber/assets/scss/components/_hero.scss
from line 37 of wp-content/themes/timber/assets/scss/screen.scss
>> color: $white;
--------^
[18:51:06] Finished 'styles' after 91 ms
Make sure gulp displays scss/js error messages on the terminal when running. Example error: