Loris,

I did what you've instructed before, mixing 1 drop of OTC peroxide (3%) in it and you're right. It's stale FO. Now I got clear whites in the step from 18 to 21, the part under the cover of OHP is from 16 to 21. But other than that, the steps seem just a little bit darker and still lack of full black. The exposure time was 20 minutes and distance to the light was about 2.6 inches.

There's just one more question I'd like to ask. Is it possible to have full back using stale FO with peroxide added?

Thank you so much for the patience and all the kind and detailed suggestions:-)