Likely candidates are:
- The newspaper you are seeking isn't online at all - this could be because it hasn't been digitized, or it has, but not for the date ranges you are seeking. Check out this Directory from the Library of Congress for help.
- You need to improve your searching skills. Your ability to put together a "winning" search criteria is lacking. Keep working at it and read about different search techniques to build your skills.
But the most likely reason is below.
Take a look at the two images below. They demonstrate why you can't find the articles that you are seeking.
The one on the left is a newspaper article from the Evansville Daily Journal in 1855. The one on the right is the index that was created after the article's page was scanned and the OCR process applied. Searching for something in that index won't yield many positive search results would it?
You see, positive results depend on the index that is created after the newspapers are scanned and the OCR process is applied. It is all about the index. If the quality of the original materials, or the scanning process, or the OCR software does not accurately result in the index having the exact letters (or at least a significant percentage of accurate letters), you just won't be able to match your search criteria to the index.
Do not get frustrated - some sites do a better job than others, but generally the age and quality of the originals will dictate the quality of the resulting index. It is that simple.