We have a Kitfox 7 SS with Airstreak 26" tyres. The tyres have slow leaks due to Catheads (Three Cornered Jacks to some). I have tried to get some No Flat tyre liquid in to the tyres through the valve stem (valve out) with no luck. Using a thin stick there seems to be a "safety flap" or something just in behind the vale stem preventing the liquid from going in even under moderate pressure with an air compressor. Can anyone suggest what I am doing wrong please?