I figured it out, but thanks Tigger and Snowbird for your thoughts nonetheless.
Usually I set the bed support poles on the second lowest of the four height adjustment slots. But this time, for no particular reason, I set the them on the highest one. The bed was a little balky pulling out but I didn't think much of it. Later when I went to push it in, it stuck fast after just a short travel. I slept on it last night (not literally on the stuck bed) and today thought, what did I do different setting up this time that might have caused this. The only thing I could think of was putting the supports in the higher slots. So I dropped them to the lowest, and the bed slid right in. Apparently the higher position was deflecting the bed upward just enough to cause the slide rails to bind.
Just another of the many, many things about trailering I've had to learn the hard way. One day I'll write a book.