We just moved into our house three weeks ago. After being in the house for almost a week, we noticed that the downstairs toilet bubbled when ever we flushed the upstairs toilet. A couple days later the bubbling happened whenever water was run in any of the sinks or the upstairs shower. We also found that the downstairs shower backed up when one of us used the upstairs shower or when we ran the dishwasher. All of the plumbing in and under the house was just replaced prior to purchase, so we assumed there must be a block in the main line.
We opened the clean out and tried snaking it with a 25 ft snake but did not hit anything. We then used a main line cleaner treatment that did not seem to work (the downstairs shower still backed up when we used the upstairs shower). So then we attached a rubber bladder to the end of our water house and ran it down the clean out and ran it for about ten minutes. This was successful. We were able to shower and run the dishwasher and washer machine with out the downstairs shower backing up, and the downstairs toilet did not bubble anymore.
For about a week, we were problem free. However over the past two days the downstairs toilet has began to bubble again when we flush the upstairs toilet. We are nervous that it is just a matter of time before the downstairs shower begins to back up again. We were told by a friend that the problem might be build up on the mainline pipe. We bought Zep Drain Care Build Up Remover in attempt to combat this. We applied this through the downstairs shower drain. Do you think could prevent the shower from backing up in the future? Do you think we should apply this through the clean out? Or is there something else you can suggest to prevent future back ups?

Thanks,
Christina