I had another longer answer but I clicked off it unfortunately.
In order to better assist you please post the Information regarding your versions as outlined in Looking for technical support, please provide Plugin environment
I know that you have posted before, but as you may be aware there have been a lot of recent updates and by providing the information, it helps others when they are looking for similar results.
What , how many , where markers show are inter connected with your results page, your Zoom levels, and even the size and shape of your markers or pins. You are using big red balloons, you could try a smaller pin marker but without individual street addresses that will only work if you have unique markers per individual perhaps. Check your zoom level, is it too far out, not far enough?
After reviewing your site I see that you have many clients in one city and you are not adding specific addresses such as street name. This of course will just give you one marker (per city) because Google and SLP has no way of knowing an exact location, I assume yo have them entered with specific modifiers/ids. Perhaps you could add in more detail or add a specific marker per entry. Without a street address though I am not sure how you are visualizing unique markers. Do you have street addresses and just writing into the results layout to not show them?
I have attached the screen shots of different markers I use, both with and without cluster markers and the zoom levels.