Geco is usually very good brass.
In it's factory form, it IS loaded a bit hotter than U.S. standards, but is within NATO spec.
You may have encountered some with a bit harder than normal brass, sort them & try to run them by themselves.
I've loaded several thousand of them & they are prefered (by me) for hi-intensity loadings. I load all of my 9mm on a Lee 1000 press with an undersize sizing die (-.001), which is avaliable from Lee & don't have any problems with them other than an occasional tight primer pocket.
Use nickle plated primers, like CCI & they go easier.
A-Merc on the other hand, DO suck !!