After using inserts for many years in my golf biz and probably installing over 2000 over that time....I have come to the realization that for the majority it's not the best thing to use. Whether it's Sensicore, Prosoft Inserts or McMaster Carr 5/8" backer rod I have experienced the same thing with many of my clients and see it myself.
Why you may ask. I think it mutes a well struck shot too much and doesn't allow good feedback. On players that never or seldom hit the sweet spot on a club then yes I would recommend them.
I have stopped using inserts unless I see a player that really needs them or wants them. For players with joint pain or the need of more vibration dampening I recommend graphite as graphite has come a long way in various weights, shaft profiles, etc. I believe graphite mutes vibration much more than an insert in steel.
This is just my opinion and I am almost sure the majority will not agree.