Many aircraft broadcast ADS-B signals which tell information about their course, flight number, altitude, speed, location etc. These signals are picked up by ground-based ADS-B receivers which then feed this data to Plane Finder servers which is then shown on the site. Many aviation enthusiasts have receivers set up which pick up signals from aircraft and have kindly fed them to Plane Finder to help with the site's fantastic and ever-growing coverage. The problem which you are seeing is happening because the aircraft has flown into an area in which a receiver has not been set up to receive signals from the aircraft. Previously, the plane's information along with its location would update about every 30 seconds, but without new information, Plane Finder can only can estimate about the plane's location if you have that setting enabled in the 'Plane' tab in Map Options. But when there is constantly no new information about the aircraft, the flight estimation is no longer accurate and then the plane will 'time out' and disappear from the site to prevent confusion. The plane would have continued its flight and it may have appeared again, back on Plane Finder when another ADS-B receiver picked up its signal. A plane may drop out on its flight on Plane Finder many times before it lands at its final destination.
Plane Finder is always seeking help from people to help increase its coverage all around the world. If you know somebody who is willing to share data to Plane Finder, they could certainly write in a request for a free ADS-B receiver from Pinkfroot, the creator of Plane Finder.
I hope that this helps you to understand your problem. If there are any questions, please don't hesitate to reply.