If the nickel gets above the melting point of the solder, it should flow nicely. You need to have good thermal contact between the tip of the iron and the nickel. I press the tip against the work and feed in a tiny bit of solder to act as a heat bridge until it heats enough to flow, then feed in more solder. You should be able to melt the solder a few mm away from the tip if the nickel is hot enough.
You can also get the iron too hot and it will burn off the flux before it has a chance to work.
"One test is worth a thousand opinions"