Make your torrent client programme connectible. Requests from other member's torrent clients don't reach yours.
Then what happens is that at the first sync with the tracker when you start seeding, there isn't of course an leecher. If are leecher joining, they'll get your IP from the tracker and will send a request to get data, but it doesn't reach your torrent client programme if it is not connectible.
The next sync with the tracker initiated by your torrent client programme, it will get all the IPs of the other members trying to download. From then, your torrent client programme will start to propose uploads to the downloaders and traffic will start.
The easy and dirty work around is to Stop (not pause) the torrent when you see a leecher on the site, then after a few seconds to restart it. That will initiate a sync with the tracker outside the 30 minutes intervals.
The correct solution is to make the torrent client programme connectible. Usually it are the forwarding (aka NAT = Network Address Translation) and the firewall settings which cause the issue.
µTorrent/BitTorrent got a built-in test: Options > Setup Guide > select the "Network" test and run it. The result should be a green check sign, not a yellow/orange only.
a) A easy solution for the portforwarding, is to activate UPnP and/or NAT-PMT in both your torrent client programme and your modem/router (the box from your ISP, usually it is already on there). Then restart your torrent client programme
b) The firewall needs to have an exception for incoming connections for your torrent client programme on the port which you've set to be used in the torrent client's connection settings. There may be multiple firewalls active, e.g. Windows' built-in firewall, your internet security or anti-virus built-in firewall. All need to have that exception. Easy try is to set the Windows firewall exception to "On" if your torrent client got such option and restart the torrent client.
