It shouldn't be this hard - Clogged drain

My tub/shower drain is clogged and the drain makes it hard to plunger. The drain has a cover with a knob on top. While the knob comes off, the cover doesn't. How in the world do you get this cover off?? I have a Delta system. Also, I have already tried to snake through the clean out...I have not had great success doing this.

While hard to see, there's usually a setscrew under the cap holding it to the shaft. Pull it up as far as it will go and maybe use a small mirror and light to locate it.