Ian, a couple of questions:
1. What's the temperature and relative humidity of your darkroom / paper storage? (Too dry is no good for coating paper...)
2. Have you tried brush coating? (Just to see if you get something different...)
3. Do you use any surfactant? (Tween, Ilfotol or similar...)
BTW, 20 drops per 8x6 sounds like too little solution to me; my usual is 0.25ml coating solution per 10 sq. in. coating area and it works great with many different papers... (8 x 6 / 10 x 0.25 = 1.2ml, 1.2ml x ~25drops per ml = 30 drops vs your 20 drops!)
P.S. I have no experience with Herschel paper, but since Mike Ware recommends it (see here) it should be a very very good paper.