I brewed my first lager on Memorial Day, a Dortmunder Export. OG was 1.053. I made an extra large starter with 2 packs of 2206 (I used a 2L flask and half gallon growler jug). I pitched right around 52 degrees and kept the temp around 50-52. Saw fermentation beginning in less than 24 hrs I checked the gravity after about a week and it was about 1.022, so I warmed it up to 63 for the diacetyl rest. That was this past Tuesday. I checked the gravity again on Thursday night and it's only at 1.020 and again today with the same result. I'm not sure what's gone wrong and why my fermentation has stopped. Any ideas?