Another toilet problem

I'm seeking some insight on a toilet problem. Searches on the internet have not matched my situation so I'm hoping someone lurking here can give me some advice.
Problem - the toilet bowl will interrmittently not fill completely after a flush.
Environment:
older 3.5 gallon flush toilet
identical toilet on opposite side of wall flushes OK, they share the same drain and I assume the same vent
the tank fills normally (at least all the times I've checked it)
I think I have ruled out the fill valve and the flapper valve (the easy things to fix).
I don't think the vent is plugged otherwise I'd think the other toilet would exhibit the same problem.
And I don't think the drain is constricted or again the other toilet would exhibit the problem.
I'm wondering if the fill holes around the top of the bowl are getting constricted (but then why is the problem intermittent??)
Any sage advice out there?
John Keith snipped-for-privacy@juno.com
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Is it filling at all? After a flush there is still some water in the bowl without any refill water coming in. If the float valve is sticking it may not be turning on the refill valve so you are not refilling. It may also be only partly turning on that valve. Second there is a possibility that there may be some obstruction causing a late full flush.
I would not rule out the possibility that the vent or drain may be partly blocked. This can happen in the winter if condensation starts forming ice in the vent. The two toilets may not be reacting the same way to a partial blockage. I know you have considered this, but I would not totally rule it out even though your theory is correct and does mean it is far less likely.
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Joseph Meehan

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Joseph Meehan wrote:

that's gone bad. As mentioned above, a late flush can cause a filling problem.
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That sounds like a clog of some kind. I kinda doubt the wax ring, but I can't rule it out. You might just try snaking it and see what happens.
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Joseph Meehan

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Check inside the tank to see whether the refill tube is actually spraying water down the overflow tube. Flushing commonly leaves varying amounts of water in the bowl and the refill tube has to supply enough to bring the level up from the lowest starting point.
Don Young
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Good Point
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Joseph Meehan

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You don't have cats and somebody who takes the cat crap from the litter box and dump it in the toilet, do you? I had a similar situation. The small amount of cat litter would build up, partially clear, then repeat until it clogged entirely.
Break out the toilet auger and have at it. Eliminate the obvious, and...
R
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"John Keith" wrote

Keith, I agree with the other to rule out the obvious, some of which you have done. I had the oddest one though once in an apartment.
Although it is very unlikely to be your situation, here's what it was. A second bathroom was added. The newer one, which happened to be the low volume flush type worked fine. The earlier one, sometimes did what you say.
The culprit was a pressure balancing valve of some sort, set to the new toilet, but on the same feed as the old one.
The reason why I didnt add it up, was I didnt account for my husband taking a shower in the 'other bathroom' when I flushed the toilet in the older one. Although the owners never fixed it (not really critical and once they put a low-flush in the older bathroom it would go away), it did have us puzzled for some time.
Once we figured it out, we let the owners know but we both made the same assumption which was it was better to leave it alone since in a year or so, they'd end up needing to replace the older toilet anyways and getting the bigger flush units was already hard by then.
It was caused by slower fill of the back tank on the bigger unit. It filled, just not as fast. Pretty doubtful this would be the same problem for you, but the symptoms were identical.
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