Sometimes the motor is designed to do that. The "brain" of the slide will sense if one of the sides is coming in slower than the other, so it will cut out. You let go of the button and try again, then it brings the other side in to even it out...may cut out again, then hit the button and both should come in at the same pace.
Not sure that is what is happening, just that it can do that by design to protect it from serious damage.
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