Hello and thank you for your questions and compliments.
Flatworms can be tricky to deal with and as you have found out what works for one person doesn't always work for the next person.
You mentioned the use of chemicals to treat for flatworms. This does tend to work for most people, but occasionally it doesn't work. It is hard to say exactly why this is. There are a few possibilities such as not calculating the correct dosage (taking into account the tank, sump, refugium or other areas of the tank), not letting the medication work long enough before turning filters back on and adding carbon or not shutting off the filter (with carbon). Sometimes it can take multiple applications before it works fully also.
But many people don't want to medicate their tanks and would prefer a more natural method of ridding their tank of them. The sixline wrasses are one option, but as you have seen they are not a 100% viable option. For some they work great (I personally have used them on two occasions and both times they did the trick) and for others they don't touch them. Sometimes this can be due to the feedings of the tank. If you tend to be a heavy feeder, they may never touch the flatworms simply because they are getting their food from a different source. If this is the case in your tank you could try reducing the feedings to see if that helps.
Then there are the Velvet Sea Slugs (http://www.marinedepotlive.com/velvet-sea-slug---chelidonura-varians-inverts--nudibranch.html). These guys like to eat flatworms, but are not the hardiest of animals. They are hard to get a hold of and if you do find them many times they don't survive long.
Flatworms generally will be found mostly in lower flow areas that get a good amount of light. Increasing the flow in the tank and trying to reduce any areas of low or no flow in the tank can sometimes help. In addition siphoning out as many as you can on a daily or weekly basis can help keep their numbers under control and hopefully then a wrasse or medication will be able to help get rid of the rest of them.
While denitrators are not overly popular many people do use them. Marine Depot carries the Korallin BioDenitrator (http://www.marinedepot.com/ps_AquariumPage~PageAlias~filters_korallin_biodenitrator.html) and it has sold very well for them. Along with denitrators there are many different types of filter media that can help reduce nitrate levels. Water changes are also a great way to help keep nitrate levels in check.
I hope that helps some with your flatworm problems. If there is anything else we can do for you please let us know.
Marine Depot Customer Service