I've had the best two months subscriber-wise in Feb-Mar that I've ever had. 98 subs since 1 Feb.
That said, I agree with @CarltonIsaac - you promote on the forums a lot but there's just not that many people here. Once we've all subbed, making new sub4sub posts can't really help. You have to find non-Tapas places to promote and bring readers to you. I don't know how you do that with a novel, but I post my comic behind schedule on lots of other places and that helps a bit!