Nether portal

Randomly when me or my friends go through a nether portal either from the overworld side or the nether side the game doesnt change to coordinates. 

Example. Overworld cords 100, 50, 100 when going into the nether it will still be 100, y, 100 even though I'm using a portal that has been active and working for months. Same with coming back from the nether. Either way it will spawn us in and there wont be a portal near us to even go back through. Happens randomly.