CD-
Different people float differently. I'm a cork, add a heavy neoprene suit and I need an incredibly heavy weight belt, don't ask. Dunno about the buddy. No, they didn't drop weight, let's just say there was a problem and the solution was for me to get us both out of the water (to the surface) in a convenient manner. Buddy's BC was not inflating, either--similar to being in the water wearing a PFD and finding you have company who isn't wearing one at all--for whatever reason. (Bottom line was "pilot error" not a gear failure, fwiw.)
Bear in mind that when you are SITTING on a PFD, you are getting nearly 100% bouyancy from it because it is fully submerged. Now, WEAR one. The half of it that is on you chest and shoulders is not providing any bouyancy at all, it is no longer submerged.
And every inch, literally inch, that you can get your mouth and nose above the water level, helps to ensure that you won't inhale spray and drown in storm conditions. (What works in flat water just won't work as well, say in a two foot chop or a 30-knot wind with driving rain.)
Which is why spray hoods were invented. I suppose one day I'll have to update my PFD to catch up to that.<G>
i2f-
If that vest didn't stay substantially inflated (not stiff, but full) for a week, there might have been a leak, or some crud in one of the valves, making it leak. Shouldn't need more than a puff every day to keep it inflated for the week. We used to regularly test BCs the same way, blow 'em up from the tank connection, set 'em aside, see if they'd last multiple days without wilting. And if they wilt--something needs looking at.
Which is probably the most valid argument against inflateable PFDs, they can't just be kkept and used, they need to be inspected and maintained.