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Poutnik is right, what likely happened is one or both of the reagents lost its potency through chemical reactions to which they are susceptible.
In the case of the sodium hydroxide, if it reacted with carbon dioxide from the air (it is best stored and handled under a CO2-free atmosphere), the resulting carbonate is no longer a strong enough base to react ...
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Your questions (or complains) are encompassing human knowledge acquired over a century and may be more! Science has several collections of known facts, huge body of information and knowledge. You have to know them just like in math you learned 2+2=4, and the area of the rectangle is length into breadth, and the ratio of circumference of a circle to diameter ...
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pH measurements at extreme pHs are not accurate. It is not a surprise that you find unpredicted changes in pH after bubbling carbon dioxide because the sensor is not meant for such a high concentration of NaOH. There are well known sodium errors associated with glass electrodes. The sensor is not faulty, it is not designed to be used in concentrated alkalies!...
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