Joe,
I've got a couple 66's and 99's. One of my 66's came to me missing a slide plate. One of my 99's needs some work so I borrowed the 99's slide and put on my 66. I've come to realize why my 99 was missing it's slide: the spring is weak - won't 'hold' the slide. I tried prying up the spring but that didn't work so I bought a new spring - which is why I suggested a new spring be ordered at the same time. Her spring was probably just too loose or weak to hold the slide.