One spot on a plywood does not accept stain. What can I do?

I bought a plywood from lowes for 49 for a table top. It has a smooth finish and has an almost white color. I sanded it with 150. Applied minwax pre stain and then dark walnut stain. All oil based.

Then I noticed few cross grain scratches after staining. I sanded the scratches over the stain until they disappear. Then I tried to re-stain the spot. But, now it does not accept stain. I tried to soak the spot with stain still did not work.

Later I removed the stain from the whole surface with thinner and then sanded it with 150. This time I applied Varathane water based dark walnut stain. This worked bit better on the spot. I even tried washing with soap and water to remove grease if there is any. But still it does not accept stain well. Any idea what is going on or what I can do?