Our toilet on the second floor is possessed ! Sometimes it flushes just fine and sometime it won't at all unless aggressively plunged. It has been getting progressively worse over the 4 years we've been using it. We can flush it with absolutely nothing but water in it and get the same result. THere is no pattern to when it will flush and when it won't.
Here are the details:
it has a 4" drain and the toilet itself is a regular American Standard 1.6gpf. My husband has pulled the toilet and found nothing in the toilet trap or the drain pipe, pulled the vent stack at mid point in our attic and looked up to the sky and as far down as possible and found nothing and opened the clean out in the basement while I've flushed and watched water rush by. The septic tank was pumped in November so we know that's not full.We've tried pouring water in the tank and directly in the bowl : same inconsistent result. We've also cleaned out the holes under the rim to make sure no mineral deposits are there.

We are in the process of building this house (a 10 year process!) and the first floor toilet is not yet installed. There is just a rag stuffed in the 4" pipe to keep gases from coming in.

The two toilets are in a straight line (stacked bathrooms) and share the same drain and vent system. We're stumped.

Is it possible that somehow when we flush on the second floor we are gradually sucking in air through this 1st floor rag filled pipe until after a few flushes we are creating a vapor lock (or something similar) in the 2nd floor that is only "broken" with plunging? Any ideas would be greatly appreciated!