Sims won't enter house, can't place stairs or foundation or sidewalk?

I cannot figure out how to get my Sims to go inside the house. They won't use the door, just keep giving exclamations. I thought maybe they couldn't get up there, but the game won't let me place stairs nor lower the foundation any further than the base level it already is. I'm using the Island Life DLC and the house is on sand. It also won't let me place grass on sand nor walkways. What am I missing?