How can I lock up loose hair in the middle of a dread? - Dreadlocks FAQ's
If the dread is less than a week old and you already have loose, un-knotted hair in the body of the dread then the backcombing left something to be desired, don't worry though it's common to have a few dreads that need to be restarted. It good to catch them and fix them early on since you don't loose much progress.
If the dread is older and still has good knots - or the straight un knotted hair is limited to a smaller area do not re-backcomb. Instead use circular rubbing (aka pinch rubbing) to create knots in the loose spot.
This technique is very similar to clockwise rubbing, but rather than gently rubbing the loose hair between your scalp and your hand you will rub it between two hands and the knots will be created between them.
Each hand will hold the dread just about a quarter inch from the loose area. Pinching the dread between the sides of your middle finger and your "don't point, it's rude" finger works pretty well. For this reason Amy calls this "Pinch Rolling". She's a big fan of this technique. Your palms will face each other and the loose hair will be lightly pressed between them. Then rub your palms in small circles knotting the hair.
Much like clockwise rubbing you want the hair to be wax free and freshly washed and dried. Having Locking Accelerator dried in the hair helps but Lock Peppa will make the biggest difference. Just pinch the loose hair with peppery fingers before you begin rubbing.
After you've created a puffy wad of knots in the straight hair your work is done. If you've created a lot of new knots that need to be compressed, finish up by adding a maintenance serving of wax and palm rolling the new knots to compress them.
Occasionally someone will have some knots at the roots and some knots at the tip, but the majority of the dread will be completely straight hair. This is pretty rare but when it happens the best coarse of action is usually to comb out the knots at the tip and re-backcomb the dread even more than a couple weeks old. If there are knots at the root you can leave them be and continue knotting the hair from the original knots to the tip. You are pretty much starting the dread over but if it was straight hair you're not loosing much progress, so no worries.