Could the banner be giving the wrong impression? Is the content or whatever is on the banner still be seen as accurate in relation to what’s actually on the site? If it is, then can’t see why it would change anything in terms of getting better ratios.
Sites seems to have their ups and downs, some stay fairly stable other go through phases of being new and fresh then probably gets boring to surfers.
Just take at some new adult sites, they may look great but sooner or later those sites will become over-advertised and if surfers get the feeling that they already seen it… they will probably be less likely to signup.