Best answer by Navid
It looks like you've used up all 180 minutes of upload space allowed for free accounts. This is why some of your tracks are now hidden and you cannot upload more.
In order for all of them to appear on your account, you can either upgrade to a Pro plan or delete some sounds to make space on your account for new uploads.
You can check out what tracks are hidden on your account here: https://soundcloud.com/you/tracks. To see the hidden tracks just scroll back to the oldest tracks in the list and you can see they are greyed out, these are the hidden tracks and you can delete them in this view.
To check how many minutes you have used and have left, head to the upload page (soundcloud.com/upload) and look underneath the upload tool. You'll see a counter that shows the current minutes available within the account.