Hi george
There are 3 main factors that will affect the speed of sales
- quality;
- being able to identify exactly what all customers want;
- being able to tag so that when customers search, they'll always find your work;
Of all the factors above, quality is the easiest, and even that takes time to develop. The other 2 are extremely difficult if possible.
In addition, it's thousands of photographers, uploading millions of work; many of whom are more popular than you are. It follows then that what you need to do is to keep uploading your best work, tag well, and hope that in the process some of your images will get found, liked, and suitable for the project planned for the image purchase. This will take time, and dedication. As you keep uploading, and improving on the quality work, you'll become more popular, and increase the likelihood of more sale frequency.
Regards
JG