Fourth time around, just a question.

Did you go to their forums and see any of the hundreds of threads from others that are knocking on the doors?

Your images look good from here.  But that's not what they asked for.  Three very different images, vary the subject matter.  That's repeated over and over.

Check out their forums, a lot of pinpointed advice there.

But as far as warning people, well, there's no real warning. They asked for a certain type of submission, you didn't follow their rules, so they declined you.  Try again in thirty days.  Happens all the time. 

I would try to do some isolated images, maybe even of the sport equipment in the pictures.  They just want to see that you have a variety in your portfolio. 

Good luck with your next submission!

When I joined iStock I had a more or less similar experience. I submitted three photos, and got a message that my application was not accepted because the photos are or similar jubject matter, although none of the three photos was rejected. I was asked to send three other photos with different subjects, no waiting period. I send three other photos immediately and my application was accepted within 15 minutes.

It's not just the subject matter it's also the type of lighting used. They are all the same - outdoors bright sun, pretty harsh shadows on two out of the three. Not only does this not show that you can use different types of lighting, it's also a lighting scenario not much loved by istock inspectors - only the muscle man one looks like much thought has gone into controlling the image lighting. (and that's an awkward crop with just the edges of the structure he's doing the pull up on showing - better to have come in closer and lost those distracting elements)

They aren't bad photos - but they don't show that you have a half way professional approach to taking images -  two out the three look like decent family snaps with rather distracting backgrounds rather than stock images. Now as one who has best sellers which shamelessly exploit my family snaps, you can get sales on microstock with this approach. But you have to take the snaps with an eye for stock, and to get in you need to be able demonstrate that you can do more than that these days.
If you are going to use outdoors sunlit shots, think about using a reflector to reduce the harshness of the shadows. Open up the lens to throw distracting backgrounds out of focus; think about the angle of the shot. Lie down on the grass and look up at the  kid jumping, so that you fill the frame with the kid and the sky and not much else. -the shot as it is has acres of boring sky, kid almost dead centre and not big enough in the frame and a very ordinary suburban backyard still in clear focus behind. The skateboarder is not very dynamic, has a dull suburban background with distracting shadowed trees with badly blocked out details - that alone would be enough to get a lighting rejection at istock (I should know, I have a collection of them!)

I can't even remember what I used for photo application - and it was in 2006 and standards have increased a lot since then. I think there was a landscape and a couple of objects isolated on white.

The funny thing is you could make a good career out of shooting males doing sports on sunny days against blue skies. There is certainly enough variation in sport, people and photos purchased to allow for this. However it seems iStock has mantras written and abided by more rigidly than a Tibetan Monk or local photoclub judge.

I submitted one model released lifestyle, one xmas holiday still life theme, and one food. In on first try no problem. You need to spread out the range. Although the images are good, they are all outdoor, blue sky.

A friend of mine submitted stunning USA flag shot, soldier with rifle, and a really good abstract of aircraft engine. They considered it all "military". He had to play the "submit more" game and eventually got in.

Hope this helps.

Just to give an update to this, I got accepted to IS a week ago. I used one of the images from my previous application (The boy jumping), and two new images, one of a landscape and another of still life objects.

Today, my first uploaded files got approved!

Thanks designalldone! Yes , it's a start. I hope someone still finds my images there

Thanks stormchaser. Yes, it takes a while with IS' upload process to get some images online, but I'll be patient. Today I had my first rejection because of keywords, and even because they thought the signature on model release does not match the printed name! Oh well, I just need to wait a couple of weeks to explain