Hi everyone, this is my first post here! 

I am redoing our master bathroom shower and we have run into an issue with the concrete slab we created for the tile. 

We built a perimeter with 2×4’s and laid down the plastic correctly in the drain. Pulled everything tight. Mixed the concrete and made the best slope we could to the drain. Now the cement is hard and solid, no cracking, however, the far right corner seems to be floating up from the plastic. I don’t know how else to describe it other than we I step in the corner the cement will go down, it doesn’t seem really tight into the corner. 

we also realized after the fact that we goofed and didn’t put concrete all the way to the edge because the hardie baker board was not up yet. 

any help is appreciated, thanks!