I think you're doing great for someone with 75 images in their portfolio. Yes, you read that right, "75."
Many of your images are similar so when you break it down your portfolio is actually smaller than you think. Nothing wrong with similars, it increases your chances for a sale, but you will have four images competing against each other for a single sale. That's a bit different when you have four different images lined up for four different sales. Which scenario is better? It's best to do BOTH. Several images from the same shoot but diversifying with many different concepts.
Speaking of concepts, this is a STOCK agency and it's not the same as a MODELING agency. Concepts sell better here, PRETTY SMILING GIRL head shots do poorly. Buyers want images that COMMUNICATE A CONCEPT, they don't want a portrait of someone else's wife or girlfriend.
COWBOY USING ATM MACHINE, now that's a good concept. Have you looked at other images in the database of people using an ATM machine? Your image is a little flat. Even though it was outdoors you might have made it pop better by using a flash.
CONCEPTS get noticed, WELL EXECUTED concepts sell. Learning HOW to do concepts, well, it took me a while to catch on, you have to figure it out on your own. There is no "couple of sentences" someone can post here that changes your eye through the viewfinder overnight.
Lastly, don't make the same mistake the vast majority do: They look at the database, say "I can do that!" and they copy what has already been done. Admittedly I do that too but you will go far if you blaze your own trail and load up your portfolio with images only YOUR imagination can conceive. Original, unique ideas go far in the stock world.
Trust me though, you are off to a very good start. Most give up when they discover that taking photographs with a high end camera, especially for the stock world, requires more skill than they think.
Nikon D800, D100, Canon G15