Usually that is a sign of too little flux, or possibly needing a slightly higher temp.

Another cause can be a dirty tip, or not wiping off the excess solder from the tip before starting the next connection. A dirty tip does not allow solder to flow well or transfer heat well. Not wiping off the tip allows the solder to oxidize and get dirty, so it does not flow well.

Technique also can play a role.

Try adding a bit more flux to the joint prior to soldering, and ensure your tip is wiped clean each time you make a connection.