For some reason I can't talk to some random NPCs. I try initiating dialogue and nothing happens. Some examples are Craddock and the weapon merchant in the Hive marketplace.

This is a common bug with a simple workaround: you need to "refresh" the area with the NPC. Either quicksave and reload or exit and re-enter the area. Sometimes you need to do this more than once. Also, keep in mind that after a while your party will get large enough that pathfinding becomes an issue - it's not uncommon when you have a full party that TNO will stop five or six times when trying to navigate around the other actors.

This has happened to me too. Just exit the area and reenter, or save and reload. I've also had the green circles & waypoint markers disappear which can be fixed with the same method.