There are three common reasons which cause tracks to skip and one less common.
The most common reason is that the tracks has been moved or deleted from their original location and to confirm whether this is the cause of the problem, you should click Tools --> Maintenance --> Verify Songs. This will remove any database entries pointing to tracks which might have been moved or deleted and provide you with a list of the tracks removed from the database. You will however, need to add your tracks again.
Two other frequent causes are either because the tracks are in an unsupported format, or the tracks are digitally protected.
The less common reason may be a damaged or corrupt database which should also be considered.