Tree roots can cause serious drainage problems. They also get worse. If this is the problem, when you plunge, you are dislodging solids, such as toilet paper, from the roots for a short period of time. The line needs to be augered and a cutter head, the same size as the pipe, used. This will clear the walls of the pipe. An auger with just a pointed haed will temporarily clear the line but not get rid of the growing roots.
if you have never made a mistake, you probably haven't done much.