I cast my line, I tried to "pull back and tap x"  The fishing guide congratulated me, but I didn't actually catch a fish.  I've tried a few more times and I can see the line getting pulled by a fish, but when I tap x it just keeps showing pulling and eventually the line just comes back to me.  Is there something I need to understand about "pull back"?  Would you expect to catch a fish every time?

Ok I kinda skipped through the dialogue myself and somehow did it. But I think it's a combination of moving the character whilst spamming the x button.

You have to wait for the fish to bite hard. Not the "tick tick tick" but the "bloooOOORP!" Then you have to press the action key repeatedly and very fast, in my experience. I have never lost a fish doing that.

Yes, like Nik said, you need to wait for the fish to bite down properly, which you can tell by the sound (louder BLORP sound) and also from the splash and rings in the water. It's subtle, but hopefully that helps! And yes, you need to press the action button repeatedly in combination with moving the character back until it's pulled out of the water :) Good luck!