If you are generally ranked in the top 1-3 positions and are not using sitelinks via ad extensions then your competitors are doing a better job than you, simple as that. If they are also not using them, please contact me and let me know your industry, I want in! Again, like most ad-extensions, remember to use site links wherever you can (there is very little reason not to use them). Sitelinks allow you to take up more real estate on the front page and enables you to include more information about your business, giving potential customers more choice and more targeted deeplinked options. Use them. Full stop.
What you can do to fix it: