Stouts and fermentation

[CDS]

I just kegged a batch of Oatmeal Stout that didn't hit the predicted final gravity, by a fair margin.
OG was 1.052, FG was predicted by Brewfather to be 1.013, which would have resulted in 5% ABV.
After what seemed to be an active fermentation, I took a gravity reading 7 days in when everything had calmed down and got a reading of 1.020. Fermentation temp was a steady 66F, so I boosted to to 68F for the second week, but after 14 days, the FG was the same - 1.020, which results in a 4.2% ABV. I don't mind the lower ABV actually, and it tastes great even before conditioning, but I am curious about what might have gone wrong.
A couple notes:

• I typically end up with a lower FG than expected with almost every ale I brew
• I always use a starter
• I always aerate the wort

This has happened to me before when brewing stouts, so now I'm wondering - is it just a coincidence, or is there something about stouts that might account coming in at a higher than expected gravity?

Here's the recipe (BIAB):

Predicted Original Gravity: 1.051
Predicted Final Gravity: 1.013

Mash:
Temperature — 154 °F — 60 min
Mashout — 168 °F — 15 min

Malts (5.12 kg)
3.4 kg (66.4%) — Crisp Finest Maris Otter Ale Malt
680 g (13.3%) — Oats, Flaked
240 g (4.7%) — Crisp Chocolate Malt
230 g (4.5%) — Bairds Black Malt
210 g (4.1%) — Biscuit Malt
180 g (3.5%) — Briess Caramel Malt 60L
180 g (3.5%) — Briess Caramel Malt 80L

Hops (68 g)
40 g (24 IBU) — East Kent Goldings (EKG) 5% — Boil — 60 min
28 g (9 IBU) — East Kent Goldings (EKG) 5% — Boil — 15 min

Yeast
1.1 pkg — Wyeast Labs 1056 American Ale 77%

Fermentation
Primary — 66 °F — 14 days

 

[VikeMan]

Stouts typically use a lot of not very fermentable malts/grains. It may be that Brewfather doesn't by default take grain bill into consideration when predicting attenuation. Most software, including the big kahuna Beersmith, doesn't.

I loaded your grain bill, mash temp, mash length, yeast strain, and OG into BrewCipher, which predicts an FG of 1.017, a bit closer to what you got.

 

[CDS]

Stouts typically use a lot of not very fermentable malts/grains. It may be that Brewfather doesn't by default take grain bill into consideration when predicting attenuation. Most software, including the big kahuna Beersmith, doesn't.

I loaded your grain bill, mash temp, mash length, yeast strain, and OG into BrewCipher, which predicts an FG of 1.017, a bit closer to what you got.

Thanks! This makes sense. Appreciate the reply.

 

[pvtpublic]

If it tastes great, then I would say nothing went wrong. I also second what VikeMan said.

 

[BrewMan13]

Plugged your numbers in and I think the issue is that the diastatic power is a bit low - getting 33. Bare minimum is 30, and I personally add a little enzyme to help if it's below 40.