solder not adhering to lead

Yup, gots a bit of oxidation, maybe, on there and the brass brush isn't taking it off and the flux, (what kind?) isn't "wetting" the surface enough to tin it.

Try this, on a piece of scrap lead, DO put a couple of pieces of scrap glass on the table also, put the scrap glass together with the scrap pieces of the offending lead, if you like using the brass brush, keep doing so, (I don't , but I should), with a fairly clean tip on your iron, flux, (but not overly wet) the joint, place the end of your solder on the lead, lay the lead on the solder, and "stir" it slightly. OK, that's probably the way you already solder, but instead of adding more solder, flux it again, heating the metal to solder temp, but under the temp to melt it, until the surface "tins" and flows properly. The flux needs the heat to work, and clean. That is a bit time consuming, but it does work. There may be a film on the surface too, and not oxidation.

Just in case your manufacturer is using some sort of "lube' in their die for extruding the lead, try wiping the joint with mineral spirits on a towel with turp and whiting, figuring the "lube" may be wax base or graphite. You want to try these things in case there is a film on the lead surface and not oxidation.

I should mention I use a BIG iron when I solder wide leads just because I don't want a problem with heat sink, so look at your iron tip too.

Let's hear how it works or if I am off base.