Hiya leesaaugustin,
There are a ton of reasons for rankings to slip - a lot of which has nothing to do with an algorithmic penalty... Here are some of them:
1. Your CTR/Bounce rates are not good and G has therefore determined your site to be lower value... This is much more important than many people realize... and G tests it much more than people realize with interaction on the SERPs....
2. Backlinks from sites which once provided good authority link juice have been hit with a penalty and consequently the value of the link has deteriorated... Or it could be you have lost some backlinks from such sites...
3. Other sites are performing better than yours....
Things you can do:
1. Main priority is to focus on your website content and in particular on-page SEO together with engaging great content which means people stay longer on your site e.g. reduce your bounce rates! So KEEP adding content!
2. With adding content - mix it up with social signals.... You may even get organic backlinks through such activities in addition to traffic...
3. With backlinking you do not need to be aggressive... I only aim for a hand full of high authority ones and at a very slow pace.... My backlinking velocity is way below the amount of traffic on my site per month.... With your backlinks you need to make sure the anchor text profile is varied significantly with commercial anchor less than 20% (my opinion due to penguin) and the rest made up with branding, urls, generic terms....
4. You could also get into video marketing given Youtube is after all the second biggest search engine...
Hope that helps!
Cheers,
Sid
