Most of the joints look dirty, as in there are a lot black bits in them. This can be down to the solder iron tip not being cleaned properly before each use. Distilled water on your sponge can help this. If the tip doesn't want to tin properly and leaves a spot that just won't take solder, de-wetting, it's best to use a new one. They can be cleaned with a small file but cheap bits have only a thin layer of tin plating which the file can remove, This will also give a patchy tip. With the tip clean and shiny, might take several tinning's and cleanings, add just a tiny amount of solder before placing the tip against the joint. When reflowing, allow to flow for a half to one second before removing the iron. Don't try and poke the solder around the joint, just let if flow by itself. Lots of your joints look as if they have been touched up a few times without really flowing the solder. Yes it does need enough heat and the tip shouldn't be the commonly fitted as stock, thin and sharp pointed one. A solder sucker is easy to use once you've got the knack and costs nothing to run except for a new tip now and then if you use it a lot.
