I have two bath rooms one with a bath tub each has a toilet . Here what I have going on some time when one of them are flushed it will not drain so we use a plunger and it will drain we have to do this with them every now and then they may work fine for a month and not work for a week . I have ran a line snake under the house all the way to the city sewer line. Check the snake at the city clean out it was clean . I do not have any tree in my yard at all (no roots in line ) But I do have a lift station at the end of my street I am one house away from it I ask the worker if there was two lines for it a inline for waste to go in to the tank and one to pump out up the hill to the main city sewer line I was told yes . Here is what I think is going on I think it is some type of vacuum but I do not know a do fix it . From the time we moved in to your home it has been this way and the clean out cap has been off . I called the health department and talked to them and the man I talked to knew my home and told me it has been that way for a long time. One more thing that may help someone help me fix my sewer the clean out that is open in the back of my home has a small amount of water in it . Just wondering the city has had a lot of problems with the lift station but when the pump goes out it just over flows in to the woods it never backs up in to your homes but I do not think this is what is wrong because I have had this problem for ever just wanted to add this . Thanks for any help .