500 images and 700 sales after four years, that's not very good numbers. You should be doing much better especially with the caliber of photography you're producing.
As mentioned, you need to address your keywording and titles. If a Buyer is searching for a picture of a cat, he is most likely going to search on CAT. Some people think they're in an art gallery and try to be fancy so they'll title their image something like FURRY FUZZBALL OF A FELINE. Images with CAT in the title are going to be ranked higher.
OK... suggestions on how to get to the next level...
You are doing a lot of concepts which is really good but I get the impression you are also struggling for ideas also. Many of your images are essentially head shots and portraits. Man-Drinking-Coffee, using a cell phone, using an ipad, those are cliche ideas. It is good to upload them but they should be secondary images thrown in after you shoot the primary images during a shoot.
So... let's talk about "Doing a Shoot." From what I am seeing, you have a few ideas here and there and you photograph them when you have time. You have access to models which is great but it appears you're thinking ahead and preparing for a shoot. You'll shoot the model drinking coffee, do a couple portraits, and that's it.
Here is the real trick to maximizing your shooting time: If you scheduled time to do a shoot several days into the future, sit down with paper and pencil and brainstorm with ideas. You have the idea of shooting a businessman using a digital tablet. Write that down. What else can you think of? You envision a businessman with his side toward the camera and looking down at the tablet. How about the model facing the camera? Then how about the model looking at the camera and holding the ipad toward the camera also?
You now have THREE shots instead of one. You just maximized your time for a particular concept.
But... WHAT ELSE? You have your equipment set up, the model standing there in front of you, WHAT ELSE CAN YOU DO?
What other props do you have? Can you throw a Santa Claus hat onto the model and have a Christmas-Businessman? Have him pull out his pockets and shoot a Poor-Businessman with no money. Do a funny picture with the businessman pants falling down.
Instead of one image you should be coming up with 10-20 images from the shoot and then weeding out the ones that don't work for whatever reason. And with a nice range of different concepts.
PLANNING AHEAD FOR A SHOOT will maximize your time and increase the number of images in your portfolio. We've all seen portfolios with a pretty girl standing next to a tree. Then it's the same picture with the girl on the other side of the tree. Sitting in front of the tree. Leaning against the tree. They upload 5-6 pictures of the same thing. They don't even think of doing tired clichés like Sad Girl, Girl on Cell Phone, etc. Realize the potential of planning ahead, create a script, and you will have fantastic results.
Concepts. You have many wonderful concepts. You also have concepts that are executed poorly. I know how it goes, you come up with an idea and find the execution a challenge to your skills and it doesn't come out the way the idea looks in your brain. One example of this is the businessman on a chess board. It was a really great idea but it just doesn't work. The men in the background don't work well in the image. The businessman shooting a sling shot, I don't get it.
I actually have a People-on-Chessboard concept in the queue right now. I think it will be accepted but it's the same problem, great idea but poorly executed. It's a little too busy and the elements within the concept aren't working well together. It's not going to sell well.
However, I have learned from the experience. I've been scripting out a set of chessboard/strategy type images and will be working on them when I have time. I think the concepts are worthy for pursuing so I will work on doing them better now that I have an idea of what it takes to create these ideas.
Does this help? I see greatness in your portfolio but I also see you struggling with ideas because you're pushing yourself into areas of photography for which you have little experience. This is a good thing because a lot of amateurs never go beyond landscapes and generic architecture, and if they get a model they just photograph them smiling and standing next to a tree. Or they just do head shots and the database doesn't need any more portraits. It's really refreshing to see someone like you trying new things.
Plan your shoots to maximize productivity and ideas/concepts. Critique yourself and learn when an idea does not work well. You see a lot of people around here who try something new and pat themselves on the back when the reality is they did a lousy job of executing the idea. (Then you see threads asking why there are no sales or views). Understand the BUSINESS of stock and understand the COMPETITION you are facing. Concepts have to be executed well and the elements need to work well together. Fixing the titles is easy to do. I think the idea of planning and scripting a shoot will take you to the next level in terms of the business. You have the skills to beat just about anyone here, even Yuri. Half of your portfolio is fantastic and demonstrates what your true potential could be yet you still resort to the coffee-drinking head shots. Time to leave the nest and soar on your own!
Nikon D800, D100, Canon G15