I started my Hefeweizen with a OG of 1.054 and used Wyeast 3068. The wort was well aerated but after 3 weeks my gravity is still 1.020 which seems a little high. The gravity reading I took a week ago was also 1.020. Could this be because liquid yeast has a much lower cell count then dry yeast? Should I pitch more yeast?
Check your brew notes, as many details as possible are appreciated and limits the speculations in answers.
Solutions differ for many causes of the stuck fermentaion.
If I had to guess, it may have got too cold, raise temp to 75°F. Or the mash was above 158° and the yeast ignored the more complex sugars.