The main thing that comes to mind for me is the colours. You have a bad mix of dirty colours combined with vibrant colours, which can make it hard on the eye.
Some possible solutions is to a) find colour pallets you like and steal from than till you develop a better eye for yourself.
B) when doing background colours subtly shift them closer intone so they blend together better and don't take the attention from the characters.
You can also achieve this by filling a layer with a base colour than lowering the opacity. (try multiply or overlay)
As well as tone down the saturation for your more vibrant colours.
