Yes, make sure you've moved the yarn between the needles and have the yarn in the correct place before making the next stitch.
To knit the yarn must be in the back of the work.
To purl you bring the yarn forward between the needles and purl the stitch. If the next stitch after the purl is a knit you bring the yarn between the needles to the back.
I'm not sure what you mean "you let the loop go free"... are you actually dropping the stitch off the needle?