I wonder if we simplify the themes then it might draw more entries. I've struggled to come up with something for some of the themes this year myself (what's even worse, I think they might have been subjects I suggested!
).
I know it's a little clichéd, but perhaps having more approachable subjects like Trees / Silhouette / Reflections etc. might mean more people participate? It still allows scope for creative interpretation, but while remaining approachable for most.
It's a lot more fun with more entries.