Have sub 2000 in 9mm. Cant lock the hammer down when trying to reinstall the bolt. Have to hold the hammer down while sliding the bolt over it. A gun smith told me that the hammer will only lock down when pressed on the 40 cal, but not the 9 mm. Is this correct??
Nope, the hammer will lock down on either, equally, they are functionally identical.
Try either a long wooden dowel, or sneak a finger through the chamber if your finger is long enough to lock it back.
Also, if you think the hammer is already locked, the bolt just might need a bit of a smack, to push it past the hammer during re-install.
And if what @Stircrayzy suggest doesn’t work, more then likely the sear spring is off. Of course this is only applicable if you opened up the 2 halves.