I got the fresh FO finally and I exposed for ten minutes first with the same workflow. And when I saw the result, I exposed for twenty minutes later. Both of them were still lack of full black.

For the ten-minute test strip, I got clear whites in the steps from 20 to 21, the part under OHP is from 17 to 21. And the shadow steps are quite far away from full black.

For the twenty-minute test strip, the same result in clear whites when using stale FO: no clear whites in the steps and the part under OHP is from 19-21.
But it's great to find that it got slightly darker in the shadow steps compared with the strip using stale FO. Yet still no full black.

BTW, I could have full black in the paper without the step tablet/OHP.

Is it that the exposure scale of the paper not long enough to have the step tablet show full black? And which part of my work flow should I consider problemetic?

Thanks much in advance for any kind suggestions.