Water filling basement window well

Whenever it rains one of the window wells fills up with water. There are 3 other windows on that side of they wall and they are all fine. The one that I have a problem with is the one in the corner. I have another window one the next corner only a few feet away and this is fine also. I know it is coming up from the bottom and not from the top because I have a well cover and the area around that stays dry. I've tried digging a little of it up and adding and repacking more dirt and stones on top to no avail. Of course, this has to happen in the finished part of the basement and the sump pump is in the exact opposite end of the basement! Does anyone have a clue as to what it might be? What I can do? How much will it cost? Thanks to anyone in advance.