I have been playing for a while now and I have unlocked all but a few levels. When I go to buy new levels it says no new levels of this type for your amount of progress. I don't know why and I am wondering if it is because I haven't completed many imperial assignments.
You will expect to spend a lot of imperial bux to unlock those levels, constantly have to rescan the probe droid screen to get kashyyyk and endor into the cheaper slots if you don't have many bux to go for the 25 bux slot. I've spent all 1400 of my bux on these missions and still haven't unlocked a single level yet, nothing but credits, imperial items I already have loads of, and the odd VIP, good luck....
In addition to the locked levels associated to the salvage missions, there is also the listed level "Panna City Medicines" which was a reward level for the late 2013 event. That one continues to appear in the album as locked and I don't believe there is any way to unlock it now as that event ended some time ago.