I had a similar problem a few years ago. The toilet, bath and sinks were all on the same drainline. The toilet was nearest to the septic system so it always worked. Periodically the other drains would be slow. The problem turned out to be the lack of adequate vents to bring air into the drains and an accumulation of toilet paper, etc around the flap inside the pipe that keeps the sewage fumes from coming back into the house. I had the drain snaked several times but it was only the use of a foaming drain cleaner every few days for a week that caused the drain to flow better. I haven't added any additional air vents yet but I imagine that would make everything flow even better.