Several have already said it, but it bears repeating: Above all else, do NOT let whether or not your photos are "popular" be the main goal.
Look, I like getting "likes" here or trending on 500px as much as anyone (okay, I admit. I probably like it more than most…

, but it's NOT. THE. GOAL.
The goal is quality, professional photos that *I* am proud to have created. Everything else is gravy (and sometimes, if it's REALLY outstanding, it's bacon.

)
You've gotten a lot of good advice on improving your photos--subject matter, composition, etc. Dig through these posts and find those things that will help you grow as a photographer.
I'd suggest using 500px as a learning experience. Pick a category that you've posted something in; particularly something you thought would "rank" better than it did. Now go to "Discover", choose Popular and then choose that category. Look at the pictures in that category that have the highest rankings. How are they different from yours? Study them and really try to objectively see how yours compare. Use that to get a better understanding of what you need to do to improve your skills.
But just for funsies, I'll also add a few things I've noticed about my OWN 500px experience:
1. WHEN you upload ABSOLUTELY matters. The day of the week AND the time of day can make a big difference in how well the photo does. Experiment with this. I once took a couple of photos that hadn't done well when I initially uploaded them. I removed them, and then re-uploaded them at times I thought might work better. They both did substantially better and one became Popular.
2. Title matters. With all those hundreds and hundreds of thumbnail images to browse, you have but a fleeting instant to grab someone's attention and make them look at YOUR photo. And you have only TWO things with which to garner that attention--the photo itself and the title.
3. Don't upload too much at once. I've found that uploading one, or at most, two photos a day is the best way to have a chance at getting a photo to go popular. MOST people have a tendency to not really go through your photo stream and "like" a multitude of photos. They click on the ONE that caught their eye. So, if you upload sixteen pictures, you are essentially competing with yourself and spreading the votes out.
4. It really does help to spend some time, especially in the first few hours after uploading a photo, "networking," as it were. I refuse to like a photo that I don't truly think is a great picture, but I *DO* spend some time in the Fresh and Upcoming categories, liking other photos that really are good--and then ALWAYS comment on the photos you like. No need to beg and do the "I voted; please visit my profile" kind of statement. I don't ask them to look at my work at all, but I find that at least half the time, they WILL go look at the photos of those who comment on theirs. Do that yourself as well--I always look at the photos of people who comment on mine. Again, I won't "like" any of them unless I truly do think they deserve it, but I always go look. It's just good manners.
Just for fun, I uploaded two photos this morning (and discovered that I haven't been on 500px in way too long).
One was an Osprey shot that is one of my better shots from this summer. The other, a sunset photo that is, in my opinion, just "nice." Not spectacular or breath-taking, just "okay." It's got some pretty swirling colors in it, which is the only thing that makes it at all attractive, imo. The scenery is just not compelling, and I didn't have an ND filter at the time to really make the sky look incredible.
The Osprey shot has already become Popular, less than two hours after uploading.
The sunset shot actually moved up faster than I expected but now seems to have stalled out at about a 58. I don't expect it to go much higher.
The point is--they have performed pretty much exactly as I expected them to, because I've learned (at least to a greater degree) to evaluate my photos. Don't expect a mediocre photo to do better than mediocre in the ratings.