Pycharm - Cannot find declaration to go to

I had same issue and invalidating cache or reinstalling the app didn't help. As it turned out the problem was next: for some reasons *.py files were registered as a text files, not python ones. After I changed it, code completion and other IDE features started to work again.

To change file type go Preferences -> Editor -> File types

I had a case where the method was implemented in a base class and Pycharm couldn't find it.

I solved it by importing the base class into the module I was having trouble with.

Right click on the folders where you believe relevant code is located ->Mark Directory as-> Sources Root

Note that the menu's wording "Sources Root" is misleading: the indexing process is not recursive. You need to mark all the relevant folders.

What worked for me was right-click on the folder that has the manage.py > Mark Directory as > Source Root.