Sounds like a wicking problem to me. Just keep messing with the density through, around or under the coil till it taste just right. Also stick with one type of wick till you figure that one out. Every different material wicks differently, have different properties and requires different densities. Till you figure one type of fiber out, don't switch up, it will just add to the confusion.
Also, boiling or treating wick material before hand is often needed to remove any oils, contaminants, or fiber taste, before using. Also helps break in the wick faster. Most new builds and wicks require a breaking in time too, before the perform at optimum. Getting to know your wick is all part of the learning curve. Personally, would stick with the cotton, pre-boil it for 30 minutes, dry it out, then start trying it again.
Or could be the rba/rda wasn't cleaned properly before using. Some have oil residue left on them from machining.