Trying to figure out what happened

So I'm just trying to figure out what happened here...

For my 5th brew, I made a PM version of BierMuncher's Black Pearl Porter. I used this recipe. Hit my mash temps fine, everything looked good. OG was 1.054, which was a little lower than expected, but good enough for now.

This was exactly one month ago. Tonight I bottled it, and when I measured my FG, it was 1.028. Bummer.

I did pitch at a pretty low temp, about 58 degrees. I really shook the hell out of the fermenter (bucket), so before I pitched there was a nice layer of foam/bubbles on top of the wort. I used S-04, and while I've always rehydrated in the past, I thought I'd throw caution to the wind here and just sprinkle on top. It took a long time for fermentation to start, almost 72 hours, but once it got going it seemed to ferment normally (based on airlock bubbling, which I know isn't really a good indication of anything - my point is only that airlock activity was pretty much the same as it's been for all my previous beers).

Anyone have any idea why the attenuation would have been so minimal? Hydro sample tasted good, so it looks like I'm going to have a nice tasting session porter, but still ... not sure what happened or what I could have done differently to avoid this problem.

Thanks in advance for any help!

From what you are saying I'm concluding that you didn't take a hydro sample before deciding to bottle. Is this correct? Also, from the way you describe it it sounds like you bottled some and then took a hydro reading. In other words, was the priming sugar in the beer when you took the hydro reading? This is the way I read it.

Try the secondary,the gravity will usally drop more.

Yes (no?), I didn't take a hydro sample before deciding to bottle. Stupid mistake, I know. While I was bottling, I took a sample (before adding the priming sugar) and put it in the hydro jar. After bottling the whole batch, I put the hydrometer in the jar and took the reading.

So, no, the priming sugar wasn't in the beer when I took the sample, and I can't secondary any more, since it's all bottled. But I did leave it in the primary for a full 4 weeks, which seems like it should have been long enough (especially for a beer like this, which wasn't super strong or anything like that) to fully ferment. Like I said, it'll probably taste fine, but I'm just trying to figure out what might have happened, so I can fix this problem if it happens again!

Was your thermometer accurate. I have missed the mark on a few, usually if my mash temps were off by a bit.