Speculation: a stratified wort when using partial boils with late additions?
eta: or dead space in the AIO that isn't reflected in the recipe calculations?
So not stuck at OG 20. Looks like actual attenuation was 72% so the yeast were probably working OK.
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What is your brewing process?
If you are brewing with a "partial boil" with late additions of extract and water (start with 2 gal water in the fermenter, add the wort, then "top off" to 5.5 gal), there is a claim that the wort is often stratified and OG measurements will be inaccurate.
I wrote this section before realizing I overlooked the "partial boil" process consideration. I'll leave it here in case it's not a partial boil.
I re-estimated OG for the ingredients listed and got OG 67.
With extract, assuming
0) the wort is not stratified
1) the recipe estimates are accurate
2) all the ingredients are added
3) the SG measurement is accurate
OG will be +/- a couple of points.
OG 54 is 72 "gravity points" short. That's either
1) missed adding roughly 2# LME or 1.5# DME
2) an extra 1.25 gal water
3) an SG measurement that's not accurate
4) stratified wort
5) or some combination of the above
In this situation, the next step would be verify the SG measuring tool is accurate and that volume measurements are accurate.