yes this is just a soldering mistake on your part. once you get some experience soldering you can easily determine what size tip, temperature, will best suit the joint. then as others have stated, tin the wire, the tab, and the tip of your iron just before attempting to solder. you want to solder quickly, and once you see the joint has turned to a nice shiny joint that's filled in sort of like a small bubble but less so on vertical joints.
also another helpful tip is to use extra flux sometimes if you're not using the correct temp, tip, etc, the extra flux can help flow the solder nicely into a strong joint. I use denatured alcohol and a small brush or q tip to wipe the flux afterward. another tip is to use heat shrink on the ends of the wire to avoid melting the jacket, though this shouldn't be an issue if you have the right equipment, temp and skill but it's still a nice touch.
if you plan to solder a lot I recommend the hakko fx888d soldering station with some different sized genuine hakko tips. this kit is temp controlled and is very high quality and makes soldering a joy.
