I have that problem, too. I'm trying to wake up later than usual so that I don't get kind of exhausted/tired when taking the actual test. I also scored lower when I took a preptest last night. I am hoping that I will get used to with taking one at night.
Pinned problem sets should show up when you go to each section of problem sets under resources. If a problem is pinned, it shows up in the Preptest to show and problem section category. Unless there is another way. Hope this helps.