alternate good flush

[lowercanada]

Does anyone have any insight into why an older, low flow toilet would alternately do a complete flush, and then an incomplete flush?
The toilet is an older bolt on tank type, and it's possible I've replaced the flapper and fill tube assembly with one that is shorter than it should (or could) be over the years. There are maker's codes in the porcelain, but no name stencil.
In the past I've drained the bowl, used a strong acid based cleanser and manually cleaned as best I could the inlets and trap-way, and it's improved the flush, but I confess to being mystified at the inconsistent behavior, so I though I might put it out for comment......
TIA.....

 

[Fitter30]

Shorter tube is less water in tank. Needs to be taller. Piece of tubing to fit over tube could raise the level . Gel type toilet bowl cleaner raise flapper pour some in watch that it comes out all the rim holes. Let sit for 4 hours to let it work on the minerals.

 

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[Jeff H Young]

good chance you have a stoppage not too far from toilet