Is it the clear 1/4" OD line that you cut to length?
These may use an inside ferrel and a compression sleeve for the outside of the line. Without the reinforcement from the inside, you may not be able to make it seal.
Or the connection line with self seal nuts on the end?
These are fairly easy to work with. Just behind the nut will be a seal. Tighten them on and the job is done.
With either of these, make sure you are not using Teflon Tape. There is no place for it with these connections.