Quoted:
your detent might need to be worked in.  Make sure the hammer is cocked and put a light amount of force on the selector, see if it moves.

Quoted:
I have assembled 2 lower parts kits and have a question.

Should I be able to switch from Fire to Safe with the hammer cocked? If so, what have I done wrong?

Thanks for the help.

Yes, you should be able to switch to "safe" with the hammer cocked.If you can't....do not attempt to fire the weapon, somethings fucked up.I would take a look at the installation of the parts OR try posting a pic here, surely someone can help you out.