There was a recent discussion about this topic that may be helpful to your problem also. In any case, one reason that observed marsh erosion is limited may be due to the fact that when marsh is eroded, land cover is converted to tidal flat. As a result, the marsh behind will not be connected to open water, and thus not eroding anymore, until the fringe of tidal flat is gone (permanently submerged or eroded away). So you may want to increase tidal flat erosion rate or change the code as suggested in the erosion issue discussion I was mentioning above.