Softened butter vs over softened butter
"Softened" butter will flex if you try to bend the unwrapped stick.  You can actually get the ends to 90 degrees to each other when you have it just right.

"Over softened" is when you can't even pick it up to try to flex it.  This actually happens before it fully melts.

The problem is that you need the butter to be at the right consistency when you're creaming in the sugar.  "Creaming" is really about cutting little holes into the butter to get air into it.  If it's too stiff, the butter just stays as a lump or maybe crumbles into pieces, but the butter never really gets any air into it.  If it's too soft, the bubbles won't stay in the butter after the sugar cuts it in.

Microwaving doesn't really help as you end up with an over-softened outside, while the middle is too firm.  You really have to leave it out at room temperature for a few hours, but if you're in a rush, you can slice it up so there's more surface area so it warms up more quickly.  (but there's the problem that "room temperature" is a cultural thing, and seasonal, so one area's "room temperature" might be too warm or too cold either all year or for some part of the year)