Why is this happening?

I'm brand new to welding. I picked up a Lincoln electric 90i a few weeks ago and have been trying my hand at gasless flux core (0.030 wire).

I'm trying to make a a little end table, but after grinding down my welds I end up with little potholes. I've tried filling these with additional welds, but after grinding those down the potholes remain.

I also noticed that the additional round of welding has really pulled everything out of square (I don't have a wedding table yet, so I'm just on concrete).

I'm not sure if it's my settings or technique, maybe the tip needs to be replaced?

Any advice is greatly appreciated!