I have been joining metals for forty years. All kinds of metal. Copper and brass plumbing, steel in ships hulls, and silver and gold. Consistently through out the fit, cleanliness, and heat are the big factors in a successful joint.
Heat,. Too cold and nothing flows,. Too hot and the out side of the metal reaches the right temp before the interior and you get a hollow joint.
Cleanliness: Shower may melt but it won’t flow through oil or grease or hand cream. The flux won’t cover the contaminant.
The fit:. Solder sucks out loud as a filler. Capillary action will pull the melt to the tightest space.
I know this stuff because my Dad hammered it into me. The fishing boats that went to sea might have sunk with less care. And my wife would have me head if the plumbing leaked behind the sink.
All that being said,. I have to learn how to solder all over again about once a year. My Dad said he did as well. We developed a process in the way we work. Almost a ritual. Anything that changes the ritual can cause a hitch. It happens to everyone of us in greater or lesser degrees. I’m telling you it happens to me. Soldier on with the soldering. And have fun.