Searching Gmail from Mail.app doesn't work
The most likely suspect for this behavior would be in how you have mail settings configured. The mail account will only search as many messages as you tell it to pull down from the server. Go to settings, and then select "Mail, Contacts, Calendars."Scrolling down will reveal the different settings that you can configure for mail. One of them will be an option to let you change the number of emails that are pulled from your server. If you get a large number of emails, but the settings are only configured to store the most recent 100, then it would be reasonable to expect search to find only those items from the last couple of days. Depending on how far back you think that you will need to go on a regular basis, the number of emails that you will want to hold onto will vary greatly. So, you can easily adjust the number of emails that will be kept and will be able to be searched, but it will be up to you to determine this based on what you think your needs will be.