How can I tighten a loose area in the middle of my dreadlock? - Dreadlocks FAQ's
You'll need to create some new knots within the dreadlock to tighten up the loose areas. Creating knots in the middle of the dread is a bit more awkward that creating them at the roots with clockwise rubbing but all the same principles apply. You'll use a variation of clockwise rubbing to get the knots started and palm rolling to help them compress. Since you can't rub against the scalp in this case you'll grab dread by the dreaded parts on either side of the loose area. You'll have about 1" to 1.5" of looser hair in between. Then press the dreaded parts against each other creating slack in the loose un-knotted hair. It will bulge out to the side of the dreadlock. You can lightly press this hair between the fingers of both hands and roll it around in the normal clockwise circular pattern. Try to gather it together as much as possible so it has the best chance of forming knots with itself. Imagine clockwise rubbing between two sets of fingers rather than between one set of fingers and your scalp. You'll want to check out the regular clockwise rubbing dreadlocks page since the prep for the hair and the goal is the same.
After you have a puffy wad of knots you can palm roll it a bit to help it compress. Be gentle with it until the new knots have a chance to lock. It's likely you'll have to repeat in two weeks or so to create some more knots since it's difficult to make them all at once. Some will settle out and they will stay better next time around. Some people will interlock their dreadlocks when they have a loose spot. This is a bad idea since it leaves wraps of hair like scars in the dreadlocks that never full go away.