Doesn't your team have bunt defenses that you put on in situations?
Basically here is a rough rundown / rules of thumb. Others may have different opinions though.
Nobody on / drag bunt - you pretty much want to charge when they show bunt BUT here is where you need to be an athlete. If it's at you then field it and the 2B or P should be covering depending on where / how the ball is bunted. If it's not at you then put the brakes on and get back to first to take the throw from whoever - catcher, pitcher, 3B - does field the ball.
Runner on first - you're holding the runner so you shouldn't be required to charge. The pitcher should take first base side while the 3B charges to take that side. So basically you have two defenders. The goal here is to get the runner at first. The second goal is to get the runner going to second on a ball bunted hard at the pitcher or 3B. The 2B and SS should be rotating to 2B and 3B respectively. IF the ball is bunted hard at you / past the pitcher then you need to get it. This now becomes a "get over" play where the pitcher takes the flip from 1B.
Runner on second (possibly first as well) - since you're not holding then you would cover your side of the field and the pitcher will take 3B side. The 3B will hold his ground in case of a possible play to get the lead runner. The 2B and SS will rotate to 1B and 2B respectively. The goal is to get the runner going to first. The second goal is to get the lead runner on a hard bunt. If the bunt goes to the pitchers side hopefully you can recognize it early enough to get back and cover 1B. It's a tough play on the 2B to get there for the cover.
Runner on 3B and / or 2B in obvious situation where you HAVE to get the lead runner - most teams run what's called the wheel play. 1B, 3B and pitcher all charge to get the bunt early for a possible play at whatever base it is. SS will rotate to 3B early because you don't want a race between him and the runner on 2B. The 2B will rotate to 1B to make sure he's there for a play if it's thrown there. The corners sell out and don't play back at all. They crash and look for the lead out. You leave 2B completely open (some may send the CF to cover 2B).
It can get much more complicated but this is pretty quick / simple.
