I think it all l had to do with tolerances on the caps& shafts. If the ear is slightly tight and cap slightly big, it will be tough. Also depends if it's new shafts or old rusty shafts. When you're talking a few thousands interference, it doesn't take much.
While shaft ears could bend inward during pressing, they will typically try to push apart during use.
Also an excessive amount of grease in the cap can cause resistance to the cap pressing on. Grease can create a ton of pressure inside the cavity and doesn't compress well.
