Made a 10-gallon batch of Two-Hearted Ale clone a few weeks back. Split the batch between two carboys. One got very clear wort, the second got a ton of trub (I more or less just dumped it all into the carboy). Due to an accident, I likely underpitched, probably used 1-and-1/2 rehydrated bags of Safale 05 between the two carboys.
Both carboys fired up within 12 hours, especially the carboy with the clear wort which had a very good sized krauzen. After about a week, the carboy with the trub dropped the krauzen and cleared out. But the other carboy continued to bubble like mad.
After a day or two of this gap, I took a reading. Sure enough, the trub carboy was 1.010, while the clear wort carboy was 1.022. After two weeks of fermentation, I took another reading from the clear wort carboy and its gravity had dropped to 1.012, so fermentation was still going and it tasted pretty decent. But it was still bubbling and not clearing.
Now, 2-and-1/2 weeks into fermentation, the trub carboy has cleared very nicely, while the other carboy is still bubbling about every 10-11 seconds and remaining cloudy, though a bit clearer than three or four days ago.
The carboys were fermenting for the first two weeks at ~62, which is on the cool side and would explain a somewhat slow fermentation pace, and then raised to 68 over a couple of days. The cool initial temp would explain a slow pace but not the difference between the two carboys.
Not sure that I'm necessarily looking for answers (the yeast gods work in mysterious ways), but it's amazing to me that two carboys under the same conditions (except the trub, which certainly may be the cause) could behave so differently.
Lesson for me: You can only control yeast so much.