Unfamiliar fitting

I've had to pull out my washing machine and because of tight fittings/accessibility, I disconnected the water supply at this fitting. I can't re-connect it! Is there some knack or special tool?

Replace the fitting, I'm pretty sure they're not designed to be taken apart and reused.

Speedfit tee
Push down collet back into nut it will slide off pipe.
Reassemble don't tighten nut dead tight!
Push pipe back in tighten nut.
They are ment to be demountable.

Not to disagree but that's actually a Flomasta fitting, they're a JG clone and are demountable, identified by the green ring and washer. Just make sure the o- ring goes in to the fitting first, then the plastic ring, then the nut on last.

You'll need to take the plastic nut off the piece of pipe that the WM valve is on. Hold the top ring (grab ring) down against the top of the nut to pull it off the pipe, then put the o-ring back into the fitting then the green washer, then screw the plastic nut back fully onto the fitting. Put the pipe insert back into the end of the pipe and then backing the nut off the fitting again by say a turn, wet the o-ring of the insert that's now back into the pipe and push the pipe back into the fitting until it hits the bottom, then tighten the nut back down fully again. It's important that the pipe is pushed fully home into the fitting otherwise it could leak or blow off.

Slowly turn the water back on to ensure it's secure.

Thank you Exedon, your explanation and FixitwithFowler: I figured it out.