It might be your soil. If you have clay or too much sand you will have trouble with plants. I have good soil but there are still some plants that do better than others. Believe it or not I can't grow zucchini in my soil. Luckily for me my sister's soil produces an abundance that she shares with me. How about sun? If you are trying to plant in shade you need shade loving plants like hostas, ferns etc.
You can just buy potted annuals for a splash of colour. Dig a hole in the garden edge and drop in the whole pot, this lets you switch them out as the mood strikes (or if they kick the bucket)