The fillet tool won't do anything if the fillet radius is larger than the dimensions available to fillet in (e.g. fillet radius exceeds height or width of the edge you are trying to fillet.. Start with a small radius setting like 0.02 or 0.05 and zoom up close to the edge to see if it is getting rounded. Then increase your radius in small increments (e.g. 0.1 to 0.2 to 0.3). There will be a cutoff value where the fillet disappears again because the radius is set too large.

Same thing has stumped me before as well.