Another silly question regarding defroster vents.

[smorg007]

Ever since I bought reproduction defroster vents and new duct hoses I cannot for the life of me get them to stay flush and in place. Its to the point I'm considering gluing them down. They have the two tabs on the side the I'm assuming were designed to hold them in place, but my problem is they don't both touch at the same time in the vent hole. It's either one side or the other causing the other side to raise up approx. 1/8 - 1/4 of an inch. I have the duct hose on as far as it will go.(It is over their protruded tab on the vent housing) I then have attempted to reach under the dash and work the duct and vent down into the vent cutout but they still pop up and out. Am I missing something. I've compared the orientation with other trucks and I know I have them installed correctly. Ie.... LH & RH & front to back.