What was total volume of wort and the ferm temp? Assuming both of those were OK, it sounds like it was just slow (various causes for that) and by racking on day seven, you may have slowed it further with the reduction in yeast count. Three weeks in primary would be fine. If you're still at 1.020 and the goal is about 1.014 (73% AA), I would warm it slightly to help the remaining yeast finish. This would also help resolve any diacetyl created earlier that the lower yeast count is struggling to clean up.
If the recipe had a higher ratio of caramel and some roast, it is possible it may be "done" at 1.016 or 1.017 b/c of the unfermentable sugars. What was mash temp?