When writing fails to communicate it's usually an act of omission. In other words, it's the things we don't do--when we fail to edit sentences carefully, or fail to clarify an awkwardly phrased idea--that cause our meaning to be unclear.
Sometimes, however, poor communication is an act of commission. A common example is when writers deliberately impede their readers with complex sentences and fancy, florid language.
Are you trying to sound extra smart? Or is what you've written just not all that insightful, and you're trying to hide that fact behind lots of big words?
I've been as guilty as anyone. When I look over my writing from my university days, all I see are semicolons and compound sentences written by a painfully insecure and intellectually arrogant English Lit student. The idea of using simple, declarative sentences was laughable to me back then. Or more likely terrifying.
Have the confidence to say what you want to say with less. Strip things out. Don't hide your ideas behind ornate prose. Let your ideas show themselves.
Don't make your readers work to understand you. Make it impossible for them not to understand you.