I've had a channel on my ER-6s stop working for two different reasons: <1> bad cable, and <2> clogged filter. Not much to do about the first problem, except repair or replace (and I'm all thumbs, so can't help with DIY). For the filter, it's a breeze to remove / replace. Your ER-6s should have come with a filter removal tool, and possibly extra filters. Both are available for not a lot of money from Ety:
http://www.etymotic.com/ephp/er6-acc.htmlIf you have the filter removal tool, but not replacement filters, remove the filter and see if it helps. If yes, that was your problem, and simply replace the filter. If not, the problem lies elsewhere.