Building Question, New Player

Hi! I'm new to the game as of a couple of days ago. I love building and chose casual infinite building for my first game. I'll play harder mode later when I get the hang of the game.

So...I built my original base bonfire -rectangular, columns on all corners, roof, stairs all around. No problem.

Then I found a place I really like. So before I get too far, I decided to start building there instead. I made the exact same bonfire base - rectangular with stairs around. Columns at the corners. But when I wanted to extend the columns to the ground, two wouldn't extend, but only stay floor height. I took out the corner floor sections and extended the columns fine. Great! But when I tried to put back the floor piece, I could not. What gives?

I had a similar issue with my house floor. Uneven ground - the only way I knew was to raise the platform by turning off snapping (maybe there are other tricks I don't know?) I had to raise each floor, unsnapped, and eye it. But there was a row in back at the correct height, only one snapped in that row, and the lower bottom row was ok. It was just three pieces in between that should have snapped at the higher height but didn't. I had to turn off snapping and eye and place each one. Why?

Thanks for any tips.