The problem referenced here used to be gone (in 7.0.3?), but now it seems like it's back in 7.1.
The problem is having a large number of (comment) characters at the beginning of a package body. When compiling that package, PLSQL Developer will instantly come back with "Compiled succesfully", but nothing will actually have been compiled.
If you get the number of characters just right (2828 characters in 59 lines, in my case), it will also truncate the package name shown in the tab of the program window.
Please look at (re-)fixing this problem. I was so happy when it was fixed before!