Ok, I did a little experimenting in the tags in my K2 site. I discovered I "damaged" some words. If I try using those specific words for my tags, the items won't show when I use those tags for searching. Those words were the first tags I had created, and I wanted to change their first letter from upper to lower case so I tried in the backend, but since I wasn't able to change the case like that I decided to delete them all and start tagging from scratch using only lower case. For some reason that was what damaged those particular words.
Now I can tag with different words, cause they will show items when searching by tags, but not the ones damaged. That's a big problem cause I need those words for tagging, they're basic words for my website's theme. I suppose that even though I supposedly deleted all my tags, the words are still somewhere inside a code and that's why I really couldn't start from scratch.
I hope all this info is useful so you can give me some good advice of what to do!