Hi,
I think you will have to be a bit more specific than that. You mentioned that you worked a lot: what kind of work did you do? Did you add a lot more relevant links? Did you add a lot of new content? Then of course, the competition for the keywords you are targeting can be another factor, as is the fact that your competition may have done a lot more and better work than you did! So, you will have to evaluate all of these factors to determine why there was no improvement in your own performance. Secondly, are there any fundamental issues you are overlooking? For example, are the pages being crawled and in the search engines' index?