Even the best made string will not ensure peep rotation, although minimal, will not occur. A poorly made string will cause erratic peep rotation, if your rotation is constant then there are things you can do to correct it and it's fairly normal to have to play around with it too a degree--it just will take time and some playing around to find that sweet spot.
As previously mentioned, you can add or remove twists to see if you can't come to an point where it is minimal(I typically remove twists). However, keep in mind that adding or removing twists may have an effect on the cables, especially if it's a slaved design. You can "walk" the peep around by swapping strands around the peep, one strand at a time. Once it gets close, you can use the d-loop to finish pulling the peep into position.