The issue is explained by Ann. You need to have your Source Patching set up before dragging any clip to the Timeline. It's not that you did anything wrong, you probably were just not aware of this important bit of the interface and the editing workflow:
Normally, source patching happens automatically in the Timeline once you select a standard clip in the Project panel.
If the clip was not shot on a video camera, and it was shot on a mobile device, a recorded stream (like a game), or a webcam video - this clip might be a non-standard one containing variable frame rates, and so on. Clips like this often defeat automatic source patching. So, you have to do it manually - just like we had to long ago.
When using clips from mobile devices, you have to be especially tuned in to Source Patching and the failure of automatic patching in Premiere Pro when dragging straight from the the Project panel (some even drag straight from the Finder/Explorer: avoid that!) I believe this is buggy behavior, but does emphasize the point that you have to be aware of this crucial UI design.
Loading clips in to the Source Monitor before editing (dragging) the clip into the Timeline gives another chance at automatic source patching. It usually works there so you can have more success in editing. You can trim clips before editing them to the Timeline by setting In and Out points prior to editing the clip, as Ann suggested.
Hope these suggestions help you lock in on source patching, a real pro feature you can now master.