Fermentation has halted

[Leard]

I'm currently brewing this:

https://www.brewersfriend.com/homebrew/recipe/view/390683/christmas-beer

I am 16 days into fermentation and my gravity has gotten stuck at 1.028. I tested it yesterday and today and it was exactly the same. I think the main culprit is the temperature not being high enough. So yesterday I moved it into a warmer room where it's been at at least 20 C. I've seen no change in gravity today however.

I have used enough yeast. It was a packet of dry yeast that I simply added straight into the cool wort 16 days ago. My FG should be 1.011 but it seems I'm a way off that yet, however 16 days is surely enough to reach that. What's going on?

 

[Ozarks Mountain Brewery]

did you have enough oxygen? is it a high flocculating yeast? did you swirl to loosen the sediment already, sometimes yeast get trapped under trub

 

[Leard]

It's Mangrove Jack Liberty Bell, so high attenuation and high flocculation. I swirled it a little yesterday, have just tried swirling it again. I also gave the fermentation bucket a good old shake up before pitching, enough that has worked for my past beers.

For the first few days of fermentation my airlock was bubbling like crazy, more than I've ever seen it before. Usually I just see small bubbles arise on the inside of it.

 

[Ozarks Mountain Brewery]

Ive had that happen and also with magrove yeast, they go crazy drop too fast and under attenuate, I'm afraid there is no fix besides adding some sugar also you might try to pitch another brand