If your procedure is quoted in some articles I don't know why you are having negative results but, Try to check if your substrate (Tyrosine) not too old (buy a new substrate for the experiment). I had obtained such négative results when using the below method with old catechol.

If there is a problem with the procedure, try the one below;

Duckworth and Coleman, 1970. The reaction mixture assembled in a 1 cm long chromatographic cuvette; consisted of 1960 μl of phosphate buffer (50 mM, pH = 6.8 at 25 ° C.), containing 20 mM of catechol enriched with an air bubble; plus 40 μL appropriately diluted enzyme; the whole is homogenized rapidly and the change in absorbance was immediately noted every 30 seconds for 3 minutes.

Thank you Dear Mr. Fouelefack. I used new pack of Tyrosine. Is it possible that it can be because of improper air-ration.

Along with fresh substrate samples, homogenization is key to this determination.

Dear Sangeeta mam,

I an working in grapes. Can you please tell me what are important thing which I should take care during homogenization.

you can also use this method.

The reaction mixture contained 0.8 ml of 100 mM potassium phosphate buffer (pH 6.0), 50 μl of aerated 5 mM (+)-catechin, dissolved in assay buffer 0.25 ml enzyme extract. The mixture was incubated at 30ºC for 30 minutes. The reaction was stopped by adding 0.4 ml of 2 N HClO4. Finally, the stopped reaction will be centrifuged at 5000 rpm for 10min. The absorbance of the oxidized catechin will be read at 395nm.