Like any SEO software, Rank Tracker sends automated requests to search engines upon checking the rankings of a website. Search engines, in their turn, seek to avoid heavy load on their servers and therefore may treat excessive search activity with suspicion.
Normally, when such activity comes from a single IP address, search engines start showing CAPTCHAs to prove the queries are non-automated; in case the CAPTCHAs are skipped - they may temporarily block the IP the excessive queries come from.
A temp block doesn't do any real harm - normally, you can resume the searching in an hour or two. Yet this can be a major holdback when it comes to regular full-scale SEO ranking checks.
Rank Tracker is equipped with a number of solutions to make your rank tracking smooth and uninterrupted, neither by the temp blocks nor by the CAPTCHAs.
If you're using the Free version, consider getting a license key to run uninterrupted checks with no need to stay at your desk.
If you're using a paid license, make sure there are no interruptions that can stop the app from using the mode.
If using the Free version, or if, for any reason, you prefer using direct connection rather than the Safe Query Mode, check out the full guide on Search Safety Settings available in Rank Tracker. These settings can help you hide your IP behind the proxies or minimize the number of CAPTCHAs you get using Human Emulation.
If you'd rather not resolve CAPTCHAs manually, consider using an Anti-CAPTCHA service (not compatible with the Free version though).