Solder Not Wetting

Electronics Guy said:

I soldered wires with a Weller soldering gun and paste flux it comes with and solder soaked into wires.

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It could be that the solder you have is not flux-cored, if it came in a it with separate flux.

You may be better off buying some cored solder intended for electrical / electronics use.

Also note that the tips of the Weller "gun" style thing can get far too hot for small electronics use and are often just too big to do fine work; you need to be extremely careful with those.

Iron tips for electronics use are normally quite small, eg. the commonest size we use is 2.3mm across, like this:
https://uk.rs-online.com/web/p/soldering-iron-tips/0542879/

rjenkinsgb said:

It's in the flux-cored solder, or they have a serious problem trying to do any electronic soldering!

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That generally works for most soldering, but if the copper is old and oxidized, a little flux applied beforehand makes the soldering a lot faster and easier.
And you can generally get by with using less solder.

crutschow said:

That generally works for most soldering, but if the copper is old and oxidized, a little flux applied beforehand makes the soldering a lot faster and easier.
And you can generally get by with using less solder.

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Even a tiny bit helps even when it's new if you're using an inappropriate tip size. Flux is great.