I think the answer lies in your question.
You generated 1000's of backlinks in the beginning which is understandable as the NHS went out and found all relevant sources to comment on and used them all, now that there are no more relevant sources left to comment on, it will of-course slow down as it cannot find any new sources to comment to.
I myself has experienced this and it is very understandable that it cant continue to make comments if there are no where left to comment on.
When you add a new site, it will take maybe a week or so to comment on all available \ relevant sites and then as it cant find new sites it will slow down.
You could try and add new links \ sites....
Maybe i am wrong maybe not, this is just my logical thoughts on the matter.
Its new sites im talking about, no matter how many keywords i include