Hi. We built our house and know all the plumbing and drain details. We have lived here 4.5 years. In the last 6 months, we have had trouble with the toilets not flushing properly. They fill up to the rim, then slowly the level goes back down. Plunging helps it go down faster, but the problem repeats with each flush. Also, the toilets (and only the toilets) gurgle when the washer drains. We snaked the drain line from the outdoor cleanout as far as it would go. No significant clogs were present. Problem still exists. Our vents are in the attic. We thought maybe the vent cap was bad, so we removed it to inspect it. The toilets all flush beautifully without the cap on. However, a new cap did not solve the problem. There is no way for birds, etc. to enter our vent system. The tubs, washer, dishwasher all drain fine. Septic was cleaned out earlier this year and "did not need it really". We do not have issues with the tanks or field per expert inspection. We have a double tank system, gravity feed.

We have yet to try snaking from the inside out, or flushing the vents because we do not have the proper snake required. We do have an internal clean-out because of a sink we never installed and it is capped. It has always been this way. Currently we are removing that cap in order for the toilets to flush as we cannot keep the vent cap off (sewer gases would build up).
I can give any amount of detail as to the layout of the drains, etc. But we did not have problems until the last 6 months. Plumbing and sewer all doen to code, inspected. Sewer lines in slab. We are baffled.