Just bought a second hand air arms 400 got it zerod at about 30 meters on a can of coke lay down front on, then pumped it back up to 200 as it had dropped to 100, tryed it again and it was way out, i had to turn scope to max and still point above can tryed about 30 shots but no change , how could it loose power, could it be that screw that you can ajust came loose , is it a special tool you need to unscrew the grub screw to get at it, and is there any seals that can blow?