These fittings have an O ring inside, and if the copper pipe is scratched or damaged it will leak.
So if you take the fitting off (push the collet back against the fitting and give the pipe a sharp tug), see if you can smooth all the scratches off the pipe with some fine wet and dry, then smear some Vasaline around the pipe before fitting it back together. It should then seal ok so long as the O ring isn't damaged.
I should know, I designed the bloody things (well not actually those ones, but the JG ones).