It's a known issue because no 2 SKXs are identical when it comes to the the position of the lug holes. For some SKXs it goes in like butter and with some it's like fighting the devil. First thing you can try is to push the spring bar inwards towards the case and use force while doing it. Second is to use your original Seiko spring bars. 3rd is to use at least 2mm spring bars, those go in immediately. It might take a bit of time to insert a fat spring bar but the flipside of all this is that once it's popped in, it'll be perfectly attached to the case with no wiggle room whatsoever or being prone to breakage.