Definitely start testing landing pages. That's probably the biggest positive change you can make.
Also, sign up for some more networks if you haven't already: testing lots of offers is key, and the more networks you can test on, the better. Don't forget to test the offers you've already tried on other networks: performance can be dramatically different.
"At the beginning It was making 1 conversion every 700 visits. It reached 5 conversions and then nothing anymore. "
"At the beginning it made 1 conversion after about 1800 visits and then nothing more."
I've the same thing as well for most of my campaigns. Is this a natural thing or is someone stealing those campaigns?
Propeller has huge inventory and unless you have a list of proven publishers to run direct linked offers, it will be quite hard to turn the campaign profitable.
I agree with caurmen, you should definitely start adding some landers to your campaigns. Here are a few reasons why you should go with landers:
1. Better understanding of traffic quality, you are purchasing.
2. Split-test different angles and designs. This way you can focus on the best performing ones as well as prevent angles and landing page design saturation.
3. Win visitor's attention right from your lander, this way they won't think twice on opting in an offer if they are already interested on participating.
In order to get your camps quickly to the green, your EPC should be higher than your CPC in which case direct-linking you don't have such insights 
My suggestion about landers: try to avoid placing your tracker's click link on <a> tags, most of those dummy bots will click those links and give you unrealistic insights about the traffic quality, hence lower the CPC per publisher. Instead, place a javascript function on your CTA button.
@johnnyx in such situations I would suggest focusing on publisher's CPC and EPC. In case it shows to be a non-profitable even after spending twice the offers payout, then pausing it won't make much harm to your camp.
Hi,
thanks for your replies. Yes landing pages are the way, but should I continue using videos and games offers or should I change vertical?
"@johnnyx in such situations I would suggest focusing on publisher's CPC and EPC. In case it shows to be a non-profitable even after spending twice the offers payout, then pausing it won't make much harm to your camp."
Ok, I understand this.
However if you get 5 conversions in the first 24 hours and after that nothing never ever is, statistically, impossible. I see this often in my campaigns and have no idea where the leak is.