Good question here Robin, I have been wondering the same, with some beetroot showing patchy germination and now the same with spinach in 2012 West Riding compost, after the same seeds germinated well in their 2011 compost. I wonder why wild rocket is being reluctant as I find that brassicas usually grow well in almost any compost. Perhaps because its seeds are so tiny (eg compared to salad rocket) and therefore can be overcome by richness.
I have a feeling that nutrient levels are a little too high and seeds prefer weaker compost to germinate, indeed such as John Innes no.1. I actually have some of West Riding’s "seed compost" which I suspect is a weaker mix and it has worked for lettuce so far.
It is frustrating to have these difficulties but growth of seedlings is strong once they are growing in the WR module compost. In all my years (sadly old enough to say that!) I have not had so many "compost issues" as recently.