Are you sitting while doing this?
Do you have your bass on your right leg?
Is your bass nearly horizontal?
If so,this will put your fretting hand wrist at a severe angle and limit your ability to spread your fingers.
Put your bass between your legs and angle the bass up.
I sit on the edge of the seat so the bass hangs below the seat.
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Raise your fretting hand elbow.
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This will straighten your fretting hand wrist and enable you to spread your fingers to accomplish one finger per fret.