There are a number of things that could affect this. To start with you need to verify that your data is accurate:
- Are you seeing personalised results (so your rankings are not what you think)
- Are you in an improved position that is likely to result in noticeable traffic improvements (top three is a good rule of thumb)
- Do the keywords you're targeting actually produce traffic?
- Are you viewing a different "version" of Google than the bulk of your audience (especially if your site has a regional audience)
- Is your traffic measurement accurate? What metric are you using?