We had stubborn drains in the upstairs bathroom. We would dump that liquid drain cleaner down the sink and shower, and it would run fast for a few weeks. Then it would slow down to a crawl. Something was letting it clog up again, or the drain was never getting fully cleaned out. We even thought it might be a vent problem. The plumbing we had in our house was not what you might call completely up to code. I went to www.plumbersbristol.org to see what we could do.
We pretty much had exhausted the options we were capable of performing. A camera inspection revealed a drainpipe fitting that should not have been used where it was. It had an angle that was not suitable for the incline of the run or something like that. The fix required getting access to the pipe. Fortunately, the bathroom was above an office space that had a drop style ceiling. The plumbers just removed the panels and opened up the real ceiling above it to get to the pipe. Since we were keeping the drop ceiling, there was no need to close up the space of the hole they made after they fixed the pipe. They just put the drop-ceiling panels back.
The drain has no been running clear for months. No more slowly building clogs because of toilet paper and other things getting stuck like sludge at the bend in the pipe. Now it is run with the right fittings. This is why there are plumbing and other building codes in the first place. Too many DIY projects were done on this house by the previous owners that were not up to code. We did not know it at the time we bought it. This is why it also pays to have a very thorough home inspection done before buying a house.