Im no expert but after my second attempt it was much easier…
I had a similar problem my first board… I used lead free solder. It was a nightmare. It’ll melt the first time heat is applied then after it cools and the oxide layer forms… good luck. Flux helps but still just avoid leadfree and use leaded solder.
Try to do the joint “right” the first try. The less you mess with it the better…
Heat the component leg and the board and introduce solder to the board not the iron tip.
Also keeping a clean tip on your iron helps. Add fresh solder and wipe it clean. A damp sponge and a copper mesh sponge are good for cleaning. If the tip is burnt and pitted, replace it.
When solder cools it forms oxides with the air. This thin layer resists heat and makes melting the joint harder. This is why people add flux, to dissolve the oxides, or add fresh solder. Keep this in mind if you’re reflowing a joint that has cooled.
Remember about thermal mass. Ground planes absorb heat and take a lot longer to get hot. Also a thin tip might be easier to get in cramped spaces but it will cool down quickly. Using a bigger tip will make it easier to keep the heat up.
These are the things I learned doing my second board. I finally got it to work after figuring out a trace that was supposed to be connected wasn’t. Diode test and the schematic helped me with this.