I think the solder thats on the board is lead free and will not easily work with rosin core. You should melt it and remove it and clean the area where you're trying to solder the power wire back on. Something like alcohol should remove any residue after you've wiped away the old solder. Also use some soldering flux.

The mamba boards tend to suck up a lot of heat which also doesn't help with soldering on them, you need a good soldering iron with plenty of heat.