Welcome to FT...

I'm assuming you've already tried full flight search, in expert mode, using "economy- no restrictions" and "miles- Y/B" selected from the fare/upgrade menus, yes? If not, try that.

I had no trouble pulling up numerous B (some Y) fares with that method, for a myriad of city pairs.

You can also check the buckets on ExpertFlyer or use one of the free availability tools and then target those flight times using the above method. Playing ual.bomb is like shooting craps sometimes...

Pat