I think regular feeding (1-3 times weekly) of larger food items (such as mysis shrimp) will help accelerate growth rate with LPS
such favia, as it provides the coral with nutrition in addition to what it is receiving via light and ideal water parameters (calcium, biocarbonate, etc). With that said, they can also thrive without regular feeding.
As previously stated, feeder tentacles typically extend at night or when food is sensed in the water (and these food items are readily captured and consumed when contacted by these tentacles).
I prefer to target feed my favia. Each individual polyp is actually capable of consuming multiple mysis shrimp. Larger food items can only be consumed when the tentacles are extended. I've actually seen my favia capture live foods in the tank as well (such as copepods and the tiny nocternal mysid shrimp which came into contact with it). Here's a pic of my favia with its short feeder tentacles extended:
Have you viewed your coral maybe an hour or two after the lights have gone out? (you've mentioned you haven't seen them extend the feeders but didn't elaborate as to when
you were observing your coral).