When I go into my settings and try to choose parts to study, I'm not able to choose the wrlting part. It actually has a line crossing it out, and the checkbox is inoperative. Have been having this problem for some time. (weeks)

Not sure if this is causing your issue, but worth a try : selecting "review only" allows you to (un)select parts to review, the "keep adding" option doesn't.

It crosses out those parts which you are set not to study in the long term. To change that setting, go to your account page, click 'study settings' and change your parts being studied there. Then you can use the temporary parts study for whatever parts you have set for the long term.