Flux is an amazing thing, but it is not a panacia of success (and can make a huge mess if not used sparingly). To use it correctly, the wire must be clean and shiny visibly, and the coil lugs or pins should be cleaned before attempting this. Wire tends to oxidize in old machines, even under the vinyl jacket. This oxidizes the copper strands and will make a poor connection unless you clean fully and use flux and a good amount of heat. The issue with soldering is it is very hard to tell the success of the connection just by seeing things 'stick' together. So many poor joints cause problems, either day-1 or day-100. Cold solder joints look good but cause resistance, and things that draw lots of current hate resistance in the flow. Poorly soldered wiring that sticks one day-1 can also unstick on day-100 due to vibration. If all is good, you will see the solder 'flow' across wire and lugs or pins and the solder will stay fairly shiny after it has cooled.
Suggestion: Practice a bit. Flux is pretty amazing, but it doesn't solve core soldering technique issues.