first thing I would do is capo the first, retune it if needed then check up the neck. If all is good, remove capo and check if the open string is out. That will tell you if you have a nut issue. Sometimes the backdraft of the nut slot is not enough and the string is resting behind the nut/neck point. If that's not it, try another string , sometimes even new ones are sometimes thinned out in areas . Then if you need to, move on to the saddle. You may need a compensated saddle or if you all ready have one it needs a different comp. There's probably a half other issues it could be but these are the most common.
Good luck