People often have a problem with this. The only thing you can do is to make sure your needles are close together at that point and that the yarn is pulled snugly as you make the first stitch of the purl, enough to make the needles touch. Hold it firmly and you do the next stitch so it won't loosten up too much.
"Trust the pattern!"
Time flies like an arrow, but fruit flies like a banana.
Hop aboard while we follow the cloud-covered van of knitting harmony, man!! Woooo!! --Hedgehog
IngridKH on Ravelry