Best answer by Mathis
Looks like you've uploaded 183 minutes of music in total at the moment, which has you 3 minutes over your total quota 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 delete click to 'view hidden tracks' and hover your mouse over the track for the 'trash' icon to appear.
Hope I was able to clear things up for you! If you continue to experience issues with this, please let us know here and we'll be happy to take another look with you.