Is this you?

Do you press up the rails to unrack?

Do you immediately start the rep as soon as you feel its out of the j hooks?

Do you feel like loose when you set start?

Take your time to unrack the bar, making sure your lats are set and you are in the right postion.

First

Make sure the rack height is set correct this is 2-3 inches lower than lockout. If its excessively low it will feel that you are doing ½ a rep to get it off the rack.
Pull the bar to the front of the j-hook. Set your shoulder blades down and back, reaching your chest up high.
Set up correctly in relation to the bar. No Spotter - Set up with the eyes directly under the bar. With spotter – set up with eyes forward of the bar (if you were to draw a line down toward the earth the bar would be directly over your eye brows)

Unrack

Create tension into the barbell towards your feet. This should feel like you are doing a straight arm pull down.
While keeping that tension simply lockout your arms.
Guide the bar until they are stable over your shoulder joint. (If done right the bar should glide into the proper position.

If the bar doesn’t glide into position you are most likely losing tension in your lats. So, keep practicing it with lighter weight and with a spotter to get a feel for it so you can master it!