What am I doing wrong????

[Cossack]

The fact that some of them fit implies length issues. Check the overall length of the case to see if the ones that don't fit exceed the max length spec listed in most reloading manuals. You will need to trim them if that is the case.
If they are within max length specs, then the shoulder likely needs to to be set back by screwing your die down as indicated by others.
One other possibility is that you have the seating die screwed down too far ( it should be up off the shell holder about two turns) so that you are partially crimping the case mouths of some cases that are slightly longer while seating the bullet