Sometimes there's dirt/rust forms inside the connection. Try spraying it out with wd as suggested above. In a kind of awkward movement for one hand, I have the tool positioned on the line, then I try pushing the line into the filter, then I press the tool into the connection. I then tug/twist on the line with the tool still pressed in while holding the filter with the other hand. I have used both the nylon and steel scissor tool as mentioned in other reply but prefer the steel tool better.

The scissor tool is made for two different line sizes.