I want to start out by saying that I don't believe that my calves are anything to write home about yet. I'm still working on them for sure. What I can say for certain is that I'm making progress. I think that there is a lot of misunderstanding about what the calf muscles are for and that might be the biggest reason that that calves are so hard to develop. If you don't understand how they work, then you won't select the right exercises to work them properly.
We are usually told that the calf muscles are used for raising the body up on the toes. This is true but it's also only a part of of what they are responsible for doing. What we neglect to realize is that the calf muscles work much like the forearms muscles do on the arm: They grip. In this case, they grip the ground. In order to get the most out of the calf raise exercises, it is necessary to raise as though you were gripping the ground with your feet and pushing your body up off the ground. Try them both ways and you'll notice a huge difference in the quality of the exercise.
The other, more neglected function of the calf muscle is that they also aid in flexing the knee. This can be demonstrated with one DVR. Place your toes pointing towards the ground. Now, tense the muscles of the leg while raising your heel up in a straight line up to your buttocks. What you'll note is a powerful contraction in the calf muscle. Often times, exercises that work the hamstrings will also hit the calves because of the flexing of the knee. Sissy squats are a great example. Just remember to grip the ground when doing them.
I came up with an exercise that really hammers the calves not too long ago by combining the movements of the calf raise and the sissy squats. It goes like this.
1. Raise up on your toes, applying Visualized Resistance to the calves while gripping the ground.
2. Once you've reached the top of the movement, bend the knees at to a right angle while keeping up on your toes.
3. Straighten the knees while staying up on the toes.
4. Gradually lower your heels back to the ground.
Gripping the ground is key. Without doing this, you won't be able to maintain balance up on your toes.
Knowing this may make developing the calves easier but it's still quite difficult. Keep your mind focused on what you're doing and be patient. It's taken me quite a while to get some definition. Persistence and focus will win out eventually.