Hi, Just joined and have a question already

I'm just casting on for ruffles and need some help... I'm a new knitter and confused about sl1 knitwise with yarn in back.... the yarn is in the back and I can slip it ok, but when I go to knit the next st, the sliped st slips off the needle...and I lose that stitch... what am I doing wrong??

I'm using bernat satin and 4.5 mm needles, I love this yarn.. esp since it's in my price range..lol. ;~)
thanks for any suggestions
Vicki