As someone who teaches soldering since years, the one thing you should realize is:

A soldering iron is not a paint brush, that means if you try to put solder onto the iron and then "paint" it onto the connection it will not work well

Solder flows to the places where it is hot enough. So if you wanna solder to form a connection between a lead and a pad you need to heat up both enough to make the solder melt if it touches that part.

It can totally be the case that your iron cannot supply that amount of heat, especially if the tipp is oxidized and it is a cheap iron.

Also: different types of solder have different melting temperatures.

Finally: keep everything clean. The iron, the PCB, the parts