One up, one back means that one player is standing at the net while their teammate is on the baseline. This is the least desirable of the three configurations mainly because there is a big gap left open in the court right down the middle.
As long as the ball stays in front of the player at the net, it is not as big a problem. But if the pattern switches and the ball is cross-court from the net player, then you are in trouble.
It also makes poaching (the opponents net player crossing the middle of the court to intercept the ball) more effective because there is a big open target to hit to when the opponent gets the ball.
Many times you will find yourself caught in a one up, one back situation. Don't panic, it's not the end of the world. But do try to get up to the net with your partner at the earliest opportunity to be more aggressive.
If you don't feel comfortable doing this because of the level of your volleys, then work on your volleys until you do feel comfortable. You will also find that the more often you move forward to the net, the easier it will become to see the lob coming before your opponent even makes contact with the ball.