Reiner Knizia is my favorite game designer. In 2002 I kept a list of every game I played, and it turned out that one out of every nine games I played was a Reiner Knizia game. And that was before Amun-Re!
But not every Knizia game is a masterpiece. Many, many of them are on my "don't bother to buy" list, and a select few on my "I really don't care for this much at all" list.
Feuerschleuker, retitled for American release as Flea Circus, seems to be quite a dud. The people who play it with five say, "Maybe it would work with three." The people who play it with three say, "Maybe it would work with five." I have a strong feeling it's not going to work with four, either. It's just too thin a pastime. Suitable for players who don't really want strategy but would enjoy sitting around picking targets and watching the luck of the draw.
Formula Motor Racing suffers from a similar problem. Each turn, you play the best card you can from your hand to improve your position. If you have the last turn, your chances are pretty good. If you don't, they aren't. There are a couple of interesting ideas in the game, but after about a dozen plays I'll be happier never to see this one on the table again.
Duel of the Gladiators doesn't even interest me enough to have attempted it. Everybody says that it's a dicefest (by which I mean seeing whether you can roll numbers that beat the numbers the other players roll is the main point of the game). They say it's a fun dicefest, but it's still a dicefest.
Exxtra is another one that just doesn't work very well. The comparison to Sid Sackson's game Can't Stop is inevitable, and Exxtra doesn't bear the comparison well at all.