It looks to me that excessive or inconveniently placed Amazon ads could be one problem for getting downgraded.
I normally write (and edit old articles) in line with HP expectations reg. desirable word count, usage of more than one text box preferably with sub-headings, insertion several relevant photos, adding a quiz (if necessary) adding a video link (if relevant) etc. Of course the basic content should be decent enough.
I don't use any Amazon or e-bay ads at all in my Hubs. I believe my articles don't go through any frequent filtering once it is reviewed after first submission or re-edit. (I believe absence of ads may be a reason for not getting downgraded).