When soldering, it is generally best to tin both sides first. Also, the parts need to mate as perfectly as possible before tinning. Solder, just like glue, needs to be thin for high strength. Don't think that you can fill gaps with solder. The trick to a neat solder or brazing job is to only clean and flux where you want the joint. The solder will only flow properly where the surface is clean and fluxed. When you heat, you heat the metal behind/under the solder until it flows and then STOP.
With solder-on barrel tenons or front sights, you need something to hold the piece in place while you add heat to allow the solder to flow together. I use a piece of 1/2" wide 5-6" long spring steel that has a 1/2" 90 degree bend in one end with a V notch in that end. It should look like a long rear sight. You then clamp the spring to the barrel such that the little V notch holds your tenon or sight in place on the barrel.
Then heat the barrel slowly below the joint (perhaps momentarily hitting the tenon or sight) until the solder flows and then STOP! You only need 500 degrees, but you do need to heat the entire joint to that temperature. Use a rosebud tip on your torch and/or move it around a lot. Don't over heat, or all your solder will get sucked out of the joint. I suspect that is one of your problems. You just want to apply the right amount of heat at the right spot for the right period of time. I know that can sometimes be challenging, but that is the goal. Soldering gun work is not like soldering copper pipes together. Pipes are a lot more forgiving.