What solder are you using? I mean the brand, number and alloy.
Lead free often gets a bad rap from old timers, but for the most part, the 217C (SAC305 or similar) is just fine and behaves more or less like leaded. The slightly higher melting point of 227C (usually Sn/Cu) isn't nearly as forgiving. Add to that there are lots of really quite poor quality brands out there with very little rosin content, or even the wrong flux core so it burns off too quickly at the temperature required to use the solder.
Specific details are important, like the tip type, solder, temperature you've set the iron to etc... Otherwise its anyones guess