I've been trying to use an Ander Lign compression fitting to connect a 3/8" OD plastic tube to an Ander Lign tee. I tighten the nut very tight, but the tube can still be pulled out of the fitting fairly easily. I've followed the installation instructions exactly. What I've noticed is that the brass insert (which came in the package with the fitting) is only about 0.17" OD while the plastic tube is 0.25" ID. Any comparable experiences out there? Is the solution just to use the next size larger OD insert so that the ferrule has something to squeeze against?

Maybe you got the wrong fittings?

Nope, it was the correct fitting, 3/8" size, which comes in a sealed plastic bag with all components.

Maybe you got defective fittings

"about 0.17" OD while the plastic tube is 0.25" what is the ID of the ring and the OD of the tube? They need to match.

The tube is 3/8" OD x 1/4" ID. The ring (ferrule) is 3/8" ID and fits smoothly and snugly on the tube. In other words, the ring and tube match properly. No, the fittings are not defective in terms of manufacture, as all components are the same sizes in several packages.
In view of no one addressing my suspicion that the insert is undersized for the tube (i.e., 0.17" OD with a 0.25" ID tube), am I to assume that you agree that this differential suggests a design defect?
Thanks a lot.

In order for compression fitting to work on plastic tubing, you need to use a tubing insert.

http://www.arrow-ind.com/pdf/Compression_flare.pdf

And I'd also use a Delrin sleave.

I have had the same problem with Ander Lign fittings - they must be incredibly tight before the can hold even the slightest pressure...