There is a lot of potential energy to be lost. The second law of thermodynamics says that this order should be lost without the input of energy. Typically, most organisms spend more energy maintaining themselves than they do to grow and reproduce. Virtually no know mechanisms for converting energy from one form to another is completely efficient. Organisms are not perfect either, and so they lose some energy every time it is transformed from one state to another.
Some energy is lost at each step, usually in the from of radiation of heat.
Equally important are the losses that come as organisms exchange energy. Plants don't trap all of the energy from the sun, and they don't incorporate all that they do trap into chemical bonds. Animals do not incorporate all the energy in the plants they consume either. The biome of the world is a complex web of energy exchanges, each of which has some degree of loss. Many of the enzymes and structures organisms have are in place to minimize the loss of energy during these transfers.