I had a bottle of RL-19 that seemed to have lost a lot of its "potency," and a buddy suggested it was due to increased moisture content. If the powder absorbs humidity, over time, a given charge weight will contain less powder and more water. Which has nothing to do with powder potency, more like a case "reversible" contamination due to water content.
So I put a packet of silica gel desiccant in the bottle and left it a couple of days, then tested it again. MV with the same charge weight had jumped ~70 fps. So I started checking it every day. I tested until the 7th day after adding the desiccant but the MV stopped increasing after 5. When it was done, the same charge weight was shooting 150 fps faster. It even was faster than my logs showed it had been when that bottle was fresh-opened, which I'm guessing indicates it came from the factory with a small amount of moisture in it, which the desiccant removed.
I have read that a little moisture actually is good for the powder, but never a reasonable explanation as to what that good it is, or why. And I lack the means to control moisture so there's only "a little," all I can do is everything or nothing, suck it all out or let it go unchecked. So now I put a packet of silica gel desiccant in every new bottle and leave it sit a week before I load from it, then test a light load to gauge its potency fully-dry. When a bottle runs out, I "recharge" each silica gel desiccant packet by putting it in a hot oven for a couple of hours, then it's ready to go in the next new bottle. So far, I haven't noticed any drawbacks to this process, and my loads definitely are more consistent, all the way to the bottom of the bottle.
Regarding potency, gunpowder was created to be a highly reactive compound, to the extreme. It even reacts with the very air around it, starting the instant it's manufactured, so its potency does decay over time, at a rate largely determined by ambient temperature. But with modern powders, the rate of decay is very, very slow. If I remember these numbers correctly, DoD discards single base powders after 45 years in storage and double base after 20 (or maybe it was 40 and 25?). And AFAIK their storage bunkers are not climate controlled, so I would imagine gunpowder stored in your home should fare even better.
All of which leads me to suggest maybe your powder hasn't so much lost potency as it has gained "water weight." Somewhere around the house, you've probably got a couple of old silica gel packets that came out of a bottle of aspirin or an iPhone or even a pair of sneakers. You could test the theory by putting enough powder for a few loads in some air tight container, like tupperware, along with the silica gel packet, and leaving it a few days, then testing it. Those "free" packets that come in so many things we buy won't suck up a lot of water, which is why I'd recommend you do an experiment on a small scale. And since that gel packet probably already is used up, it would stand a better chance of working if you'd "recharge" it first by putting it in a hot oven (I've seen everything recommended from 250°-300°F) for a couple of hours. But be aware most of those packets are sealed with hot glue, so that high of a temperature probably will cause them to come un-glued. I use larger semi-industrial sized packets that I've sewn shut with cotton thread.
And especially if you've adjusted your charge weight up over time, be wary for the potential of creating an overpressure with that dried-out powder.
. As powder gains moisture it gets heavier but it can change the burn characteristics possibly making it produce lower velocities.. Couple this with it weighing more you will actually add less powder by volume in the case if you're setting up your loads to the same charge weight that you normally do...I store my powder and primers in airtight coolers or a refrigerator...only open them when I have to..is this all necessary? I don't know but it makes me feel better 
