Hi Jani, yes as Pilou mentions the extra internal edges of your top and bottom surfaces tends to confuse the fillets.

In general the more edges that you have running together at any point, the more likely it is that the filleter will get confused. It doesn't take too much to confuse it...

I've attached here an updated version of your frame that will now fillet.

To make this version, I selected the surfaces from the top side and deleted them, and then did Construct/Planar to fill in the resulting hole with a single simple planar surface. Then repeat on the other side.

- Michael