A friend of mine gave me an InDesign file which has A-Master page that has two elements on it, but when I try to copy the whole page, it only copies the elements that were added afterwards. The background and the two elements on master page seem to be locked. As I press shift+command and click on the elements to unlock them, they disappear.
To me it sounds like the master page objects are in a layer that is locked.
There are couple ways to figure out layer it might be.
Layers panel > Unlock all layers
They could also be locked by using
Object > Lock, in which case the object(s) should have a lock icon on the left side of the frame.
In that case you can either click the lock icon, or
Object > Unlock all on a spead
Some information about master pages:
Master pages can be based on other master pages, which is handy if you have objects that are supposed to repeat in multiple master pages.
When a master page thumbnail has a letter inside, it means that it is based on that master page.
In the picture below, B-Master is based on A-Master. It's easy to tell from the letter "A" it has in the thumbnail.
You can change it by right-clicking master page in the Pages panel and choosing:
Master options for "...". Then in the new window you just change the
Based on Master: drop down list.