I've been using Rycote's Undercovers with very mixed results and was hoping someone had some insight as to what I'm doing wrong. I apply the adhesive teardrop to the inside of the shirt, then press the mic head into it. I then apply the soft, felt over - trying to ensure it is pressed down along the edges.
Yesterday I could not eliminate rustling noise. I even tried installing it a second time with no luck. I finally gave up and attached the mic with the tie clip to the outside of the shirt and out of camera frame.
I too use the overcovers, even when under the covers.
Are you sure that the rustling was friction ON your mic, rather than friction of the fabric generally, that is being recorded by your mic.
I always use a fluffy overcover on both sides of the mic, and ensure that the top of the mic is also covered by a undercover.
I also tape a generous loop of the cable so theres def no pulling on the mic cable causing problems too.