Subtraction from STL Freezes iPad/PC

Hi everyone,

I’ve been working on modifying this phone dock. However I have one cutout that I want to subtract that freezes both my iPad and my windows PC. I’ve tried this with varying sizes and every time at least one cutout will cause this. They are all of equal dimensions (copy and pasted) and all the other subtractions calculated in about 1 second so I don’t know what going on with this one.

freezegif

I’ve deleted the body for subtraction and redrew it. This fixed the freezing, but now it’s saying that it’s not a valid subtraction?