I have followed your protocol and it has worked but the staining...

I am glad to hear that our protocol worked in your hands. Yes, incubating longer (5 - 8 hours) is OK. Did you de-stain efficiently? Did you ensure that the vacuum infiltration was correctly carried out and the vacuum pressure was relatively strong (this will enable the stain to be taken up by the leaves)? Was your DAB prepared fresh and was the DAB staining solution quite homogenous? It is difficult to precisely answer your question as several aspects can impact the strength of your stain, but if you keep all these points (and of course several others in your experimental design), then hopefully you'll be able to see a stronger stain. Does your positive control definitely expect higher stain / ROS production?