Lift the finger with energy then let it drop back to the string. Practise several times with each finger to check that you have a balanced left-hand frame. Do this with other finger patterns, too.
Make sure that the finger lift and fall is part of the one cycle.
Practise this exercise without the bow to concentrate on the finger action. Then play with the bow checking that the finger returns to the same place on the string each time.
If the finger does not drop back to the correct in-tune place it means that the hand position is not adequately supporting that particular finger so adjust your hand frame. It is a common mistake to set up the hand frame to support the 1st finger and neglect support of the 3rd and 4th fingers.
Finger Action - Introduction
Scores - Preparation for Lizard Tongue - violin & viola