Revolutions are not easy to solder. You MUST have a good iron... either one with a big fat tip, or one that is VERY hot, or better yet both fat and hot.
Ha ha... seriously. If you don't have a good iron it's a nightmare soldering these motors, because of the massive heatsink that you solder the wires to.
It's also very helpful to have some paste flux handy... that will make the solder "flow"... sizzle and stick.
And like others said... also important:
rosin core solder
tin the endbell... get the endbell hot and coated with fresh shiny solder