Hi Chipp, usually my first step is to examine the "side walls" where the fillets are ending - dealing with how the fillet ends is frequently a problem area.

In your case here I notice that the "side wall" on the top is not just a simple plane surface (which is easiest for the filleter to deal with), it's a kind of wider surface. Having that did simplify topology but at the expense of not having a simple plane surface where there could be one.

If you go ahead and delete the top surface of the piece entirely (copy it to the clipboard first), just leaving a hole there, then the fillet that you want to do will succeed and then after you have done the fillet you can paste the old surface back in and rejoin it, and then fill in the hole that the fillet opened up using Construct > Planar.

So for that particular fillet that you wanted to do, it would have probably been better for that top surface to be segmented into multiple faces so that there could be a simple plane surface where the fillet ended, it looks like the place where the filleter is getting tripped up is something to do with handling the end of the fillet running into that top surface.

- Michael