i really don't think anything is up with bobby, he's just more valuable in real baseball terms than fantasy baseball terms. now that's not to say that he is not a stud, because he is.
anyways, per your request here are his numbers from the last few years:
the first thing i noticed is that his p/pa is up slightly, but he also leads the major leagues in walks this year, so that may be cutting into his production. however, it is also nice because it means he is in a position to pad his R's. also, if second is open when he walks, it also contributes to his chances of stealing a base.
his babip is a bit low this year, compared to his career norms, so that should rise and in turn bring his average up. his line drive percentage is also up again for the third straight year, which is good news since line drives have the highest chance of falling in for a hit, so what we may be seeing in his babip is that he is getting more than his fair share of line drives caught, instead of having them become hits. his gb% is also higher than it was in 04 for the second straight year, which leads me to believe this is the cause for his somewhat low hr totals, though he has never hit more than 31hrs in a year, so it does not seem that unusual to me.
all things being equal, don't really think that there is anything wrong with him, he's been playing well. i believe he's on pace for something like 120/21/120/24, thats pretty darn good in fantasy terms.