I have a Takamine P3DC12 and I noticed that the B string is lower?
First, a temporary solution. If the string is lower at the nut, loosen the string (dont take it down, just loosen it, enough to pull it out from the notch). Pull out the string, and stuff a small piece of paper or aluminum foil into the notch. Dont use glue: in case you used too much, you can remove it with a needle. Slide back the string into the notch, tune it, and youre done. A little less likely, but if the problem is at the other end, you can try the same thing at the bridge. But this is just a temporary fix. There are two ways to fix this permanently: fill in the troublemaker notch and file a new, less deeper one, or replace the entire faulty piece. Both are relatively easy, but judging by your question, it would be better to leave the decision and the procedure to an expert. It wont cost you a fortune, and a luthier will notice any other problem your guitar might have. Dont be cheap on your guitar. A good instrument can be your lifelong partner. No fancy dinners, no new dresses, no headaches, it wont even cheat on you: all it needs is a little care and maintenance. Make some nice noises together!