On some presses you have no control over how level the screen is to the shirtboard. On other presses you have an adjustment that allows you to angle the screen to the board and make it more parallel. If you can adjust your press, make sure the screen is level to the board. If your press does not have an adjustment like this you may need to use a cardboard shim on one side of the screen to make it more level to the board. The higher the off-contact the more squeegee pressure you will have to use to make the print and the more you will distort the print.
Some presses have an easy off-contact adjustment.
Others have very poor off-contact adjustments that really do little than raise the screen from the back. On these presses you need to first raise up the back clamp so the screen sits just above the shirtboard. This usually requires loosening the bolts and manually lifting the back clamp. The shim does not mean the press is not made good. It is impossible to hold the screen off contact when the press just has a rear clamp.