Could be as simple as a big ole hair ball stuck in there, I have to snake ours every 5 or so years because of that. I go through the roof vent since our inside cleanouts are cast and the piece you remove broke. Pretty cheap to rent a snake and not really all that much for a roto rooter guy either.
The not so simple reason may be that you have roots that have broken through your pipe and are now making a dam. You need a guy with one of those snake cameras to verify that and figure out where it is. If that's the problem you just need to dig it up and patch in some new line. I use "just" rather loosely, it can be a lot of digging if you don't have access to a backhoe.
Good luck