It could be many reasons. In my opinion uploading watermarked videos is one kind of free advertising. You might need to do some reseach on what the viewers want and how to convince them to your offers. Take a look around on the website that you have uploaded recently and see what kind of niches are popular. Some niches may have satured already. Also, Some site may good only for a few niches so you may need try to upload to another ones and see the result. And consider about how to place call for action to lead those viewers to your offers. Such as " Watch more at xxx, Watch full show at xxx," Write it on the desciption or just write " Visit my prolfile for more videos!" to avoid getting account banned. This will lead them to your profile page where you can write convincing text and put your url offer on it. Give them a good reason why do they have to follow your link since they can just click to the next videos and have fun. I suggest you to upload your videos to only those sites that have views statistic so you can analyze what's the viewers need on each sites.
Don't give up, You will feel a lot better when you see the first income on your dashboard. And try to get free traffic from another sources such as social media or adult forums. Those traffic are convert much better and should give you a quicker results.