This has definitely been the instance of 3D touch that seems worst optimized. It never seems clear where exactly on the edge you have to press to trigger it. A little too far out, nothing happens, too far in, you just interact with the home screen or whatever app you happen to be in. I can't get the hang of it at all.
I've noticed a couple people saying to just use the "peek" pressure, but to me, it definitely feels like I have to apply a lot of pressure to get it to activate - way more pressure than it takes to enable peek on a home screen icon, for instance.