There is no tight fit. There is a lip ( hook ) on the coping and the reverse of the same on the liner. The liner needs to be "lifted" and then put deep into the slot in the coping, then lower it. When it drops and tries to get out the 2 "hooks" will latch together. I have seen the hook in the coping flattened due to being hit with something the incorrect way, i have also seen the liner hook worn down and will not latch into the coping. The Liner Lock is GREAT stuff if used when all is new and or not damaged. Before liner lock we used telephone cable that they used inside houses, i think it was 2 pair or 3 pair as it was around 5/16" diameter and we installed it with a wooden wedge and tapped it in with a hammer.
What happens in freezing areas is that because we lower the water there is a lot of liner now above the water line. The ice usually raises the water level and in doing so lifts the liner up and out of the track in the coping. If not Lifted and dropped in correctly the lip on the liner weakens. Take a look at the coping and the liner where it is out and you will see what i am referring to. IF the coping or liner lip is damaged the soft liner lock may not hold it in. If this is the case just use wire as i said above or wooded cloths pins broken off just in the bad area and install liner lock all the way around the rest of the pool.