Yeah I think your problem is a lack of a definable hardscape. Really the only kind of scapes I see that work well without a hardscape are dutch scapes and that is because there is such a masterful transition in size, colour and shape between the groupings of plants.
I think that definitely look at getting some more hardscape in such as rocks or wood if you can. I would personally ditch the columns because I think fake ornaments are tacky, but if not, perhaps find some rocks and position them around it so that the columns look like ruins and then drape it all in something like moss or pennywort to give it the look of age. Some anubias nana around the rocks would give it a rainforest sort of feel.