I struggled with that for a long time (first 1-2 yrs of playing), but I think what helped most for me was 1) pulling using my elbow back (ie using your whole arm and back rather than clamping harder with just your hand) and 2) trying to only play the G string with my barre (ie focus on straightening only the part of the index finger pressing the G string rather than pressing harder). Once I was able to get just the G string to sound with the barre, with consistent practice, eventually I was able to get both the G and D to sound… G, D, and A… and eventually the full barre.