It took me a while to learn that heat is EVERYTHING when it comes to soldering. To little, solder will not stick, to much and you burn off the flux. The sweet spot is narrow, maybe 20F. The sweet spot changes with the size of the stuff you are soldering, with small stuff like servo wires, 620F, with large stuff like XT60, 710F, with landing gear 800F. Don’t take these temperatures as gospel, they are intended to point you in the right direction.
I started with a cheap soldering iron and my soldering was terrible. I spent $70-80 on a no-name eBay soldering station with controlled heat and a build it hot air rework station. It was like magic, all of a sudden I can solder almost anything.
The other trick is flux, most of the time there is enough flux in the solder but sometimes a dab of flux can Tuan OK joint to perfect.
The controlled heat soldering iron is an absolute need, the hot air rework station optional but is worth it. I have found many other uses for it in my shop. It’s very handy to remelt hot glue and perfect for bending plastic into useful shapes. I use it almost as much as the soldering iron.