i take it you replaced old ones, correct? if so were they straight? if they were then you are simply not getting them adjusted correctly. both the top and bottom screw holes are slotted to allow for adjustment, you just need to get them straight.

The box may not be sized for the number and sizes of the wires in it so it can be difficult to get everything to fit.

1. No matter how tight I make the screws, they slide into a crooked position. Yes I replaced old ones that were not crooked.

2. Haven't added any wires...but it's possible the receptable itself is too big.

"1. No matter how tight I make the screws, they slide into a crooked position. Yes I replaced old ones that were not crooked."

#12 wire and a GFCI receptacle in a shallow box can be a real PITA.

Turn off the circuit, pull the GFCI out, and make sure there are some bends in the wires going to the device.

It may take you a few tries to pack the wires so that the device will stay aligned, but it can be done.

I agree, some take a little work but you can do it. I think the trick would be to add the bends as Brickeyee mentions. Sometimes, the tension from the wire pulls from one side or the other. If that's the case, manually turn the receptacle until it appears straight, make sure the wire is looped so it will fold into the box as you push the receptacle in and tighten it.

I've had some receptacles that just seemed to pull themselves crooked when I tightened up the screws. I found that it helped to hold the receptacle in correct alignment with a pair of needlenose pliers while I tightened the screws. The pliers give a little more resistance to the receptacle torquing than you can provide with just your hand holding the receptacle.

Steve