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Lifting the butt off the bench typically makes the lifter stronger but should be considered an error and is not allowed in competition. It’s usually a result of incorrect leg drive. The not so telling solution is to simply play around with it. Usually good suggestions are moving the feet a little bit further back, pressing the feet forward instead of straight down, or widen the foot position if mobility allows it.
Usually a lack of full body tension, perhaps most notably the upper back.
While it might be that one arm is weaker, it's often a case of uneven foot position. It might not show on lighter weights but with maximal weights it becomes apparent. Adjust the feet so they're even. If you have a nagging tightness on one side of the body it might feel "wrong" at first but the body will adjust. Make sure to start with the new foot position on light weights and not maximal weights as it could lead to injury if you do so.
Poor technique or a strength issue. If it’s a technique issue lower the weights and practice where it's manageable. If the lifter "throws" the bar too far behind them but doesn't have much apparent technical issues it's likely weak arms and shoulders.
This is a pure lack of upper back tightness. The shoulder blades need to be kept down and back at all times and it should be practiced all the time.
More often than not this simply means too heavy of a weight for the day, or flat out fatigue in the case of higher rep sets. If, however, this isn’t the case the first thing to look for is if the lifter use appropriate leg drive. If that’s the case and the issue still isn’t resolved certain variations such as 3s pause bench press and Dumbbell bench press can be included. Even Low pin presses can be useful.
This is the most common sticking point so unfortunately it’s just a matter of “get stronger”. If there’s a need to train the particular part of the lift one can utilize Pin press or Board press at the height where the sticking point occurs.
It could be a technical issue. Pressing the bar slightly back will often help in this case. If there’s a need for a very strong lockout, doing Close grip bench press and Board presses both for low and high reps in addition to regular bench pressing is usually beneficial.