Hi Burr, yeah could you please post the .3dm model file? That would definitely help to give you a more specific answer.

Seams can be a problem for fillets if you have a cut that comes close to a seam but does not actually slice into it. But from your screenshot it does not look like you have that situation so I'm not quite sure what you are running into.

You might try using a smaller radius - often times it is surprising how much space a fillet will take up and it is pretty easy to ask for a radius that does not have enough room to fit.

That's just a guess though, if you post the model it would help to give you more specific info.

- Michael