Seems mine is barrow choice TFYKN-16
Choice is just the series name. There is also the "Smooth" and "Helm" compression fittings and the "Slim" push fittings. I am not sure of the difference between Barrow and Barrowch though unless the latter is something to do with China.
Not trying to teach you how to suck eggs but you are sliding the collar onto the tubing followed by a o-ring? You chamfer the end of the tubing and moisten it so it slides in all the way, then slide the O-ring down to the fitting and tightly do the collar up which should then grip the tubing solidly. No amount of pulling will let me seperate my tubing from the fittings so I wonder if this is down to yet another size disparity between different manufacturers of tubing? The Mizucool tubing on Ebay is a perfect fit with Barrow fittings and very reasonable too at £13.50-£13.99 for 5x 500mm lengths of 12/14/16mm tubing.