Titration of caffeine using Iodine

I was trying to do an investigation on the variation of caffeine content with temperature. I used 6 Lipton yellow label tea bags (black tea) to brew the tea solution.
Next, I perform the extraction of caffeine using standard procedures (separate by dichloromethane and evaporate). I was able to extract ~80mg of caffeine in 70-degree celsius water. I redissolve the powder into water to a 100 mL volumetric flask. I then pipette 25mL of the caffeine and added excess $\ce{I2}$ and $\ce{H2SO4}$. The chemical equation is given by: 


$$\ce{C8H10N4O2 + 2 I2 + KI + H2SO4 → C8H10N4O2 .HI .I4 + KHSO4~~~(1)}$$


$$\ce{I2 + 2 Na2S2O3 → 2NaI + Na2S4O6 ~~~(2)}$$


Yet, I was unable to get any precipitation. I carried on with the titration and found that some Iodine did react in the (1). In fact, I did get a value of caffeine close to my theoretic result. So why didn't I get any precipitation?


