'5-6 weeks old' may be the key. when i started out my dreadlocks, everytime i washed them they would improve in some ways, get worse in others. when my dreadlocks were so new they weren't really 'locked' - they were just backcombed, in the process of locking. they seemed good when i worked on them, but then they seemed to come undone a little each time i washed.
you just have to work on them after each wash i'm afraid, at least until they get more mature. i wouldn't blame it on the soap quite yet, but if you're having good luck with the other stuff you're using, continue. however i think it's a problem with the age of the dreads, not so much the soap. keep doing the root rubbing or whatever it's called and keep locking the roots.

another thing ! you should try the liquid soap instead of the bar soap, see if you like it better.
i've used the dreadhead stuff for 2 years (until today- i tried the dr bronners peppermint liquid castile soap and it was fantastic) and whenever i didn't put enough soap in my hair, it came out badly. a little slick, you could tell it didn't get the buildup of oils out of my hair. but when i just poured the stuff on and frothed up my whole head, once i rinsed it off my hair felt so clean and wonderful.
i'm pretty sure a bar soap isn't going to provide as much as a liquid, since you can apply liquid in larger quantities, so another possibility is simply not using enough soap.

so, those are my suggestions! try dr bronners liquid soap, and keep dreading ^-^

I agree with Alyx... your dreads are very young. Be patient and keep working on them all the time. And I would also definitely recommend the liquid Dr. Bronners. It's my favorite that I've ever used. It may seem expensive, but it will last a long, long time.

Be patient and give your baby dreadies the time and effort they need to mature~

~Ziggy