I recall having to either brush the tape with oil or lighter fluid or something like that prior to setting the glass which swells the tape and secures the window in the frame.

I don't recall exactly what I used (I think it was oil)...check the instructions (if any) or check the site that sold you the stuff.

The factory service manual I have for my '51 details exactly what to do on my truck, but unfortunately I don't have it handy.