Hello, ive encountered a small problem and thats that settlers turn around and leave after they gone inside the gate to my little town.

Have anyone else dealt with this?

Just to get the obvious out of the way, is it a Provisioner? They never stay - always moving. You can pick them by the giant two-headed cow following them.

Otherwise, you may have hit a limit - like beds or food. If there's no room for new Settlers, they'll just leave.

Max Settlers per base is 10+Charisma, so make sure you have at least that many beds.