Unexpected gradual color change during phenolphthalein titration of a weak acid

I titrated 25 mL of 0.05 M acetic acid with a 0.1 M NaOH solution, using phenolphthalein as the indicator. The color change was not a sharp transition; instead, it faded gradually over several milliliters of base. The endpoint appeared later than the expected equivalence point, and the pink hue was faint at first, becoming more pronounced only after a significant volume of NaOH had been added. I suspect either the indicator concentration is too low, or there is a buffering effect from a secondary species. What could cause such a gradual transition? Should I adjust the indicator concentration, or is there a fundamental reason tied to the acid's pKa and the indicator's transition range? Also, could the solution's temperature or the presence of dissolved CO₂ be influencing the result? Any insights would be appreciated.

I tried to keep the solution well mixed and used a clean burette, but the problem persisted. I'm not sure if the issue lies with the titrant concentration, the indicator, or the experimental setup. Any guidance on troubleshooting this would help a lot.