If the plaques in the spot test are small, it might be that they will be too small to see in the double agar plate which has more turbidity to it.

Dear Dr. Benedik

Thank you so much for your answer. Yes they are small. But other researchers could do DAL withe this phage. I am doing the same protocol but still nothing. Do you think if I add less agar I may gain plaque? I add 0.4 percent. What about just 0.2

You might try to change conditions, be sure your plates are fresh and very wet. Ensure the cells you use are also fresh and mid to late log phase cells. Perhaps decrease the volume of the top agar layer as well.

Dear Dr. Benedik

Thank you so much. I add 3.5 ml of top agar with 2 mM cacl2.

what is your infection protocol before DAL?

having fresh wet plates are important to nice size plaques

Dear Dr. Datta

Nothing. I just dilute the 100ML sample.