Andrew,
The Lot number is on top of the lid, P followed by 8 numbers e.g. my new brass is all P00701403.
The reason I suspect either a bad batch of brass, or (less likely) velocity being higher than you think is that I used a 6.5-284 for about 13,500 rounds with 140-142gn projectiles at velocities between about 2900 and 3030 fps, and only ever had rare instances of loose primers. Sticky bolt lift and work hardened necks were the main reasons for discarding brass.
I'm reasonably happy with the current Lapua brass. e.g. yesterday I did some velocity testing with a new 7mm Shehane Bartlein to see what range of velocities to load for ladder testing. The projectiles were 180 hybrids jammed, in my experience the hardest on the case heads. I re-used two cases alternately with 9 shots each, rising from about 2700fps up to about 2970fps. At the end of the process both cases had primer pockets that I would not hesitate to use again, and again. I know that 2209 is very highly regarded for low velocity spread, but can't help feeling that you pay for it big time with case life. BTW I fired 3 shots at 60gn of N165 with velocities of 2952, 2958 and 2953fps - far too small a sample size, but its got me hoping for a nice node in that velocity range.
So if your brass is the same Lot as mine (P00701403), then I can only think its unlikely to be your problem. Maybe you have a fast batch of 2209, a throat that's too short, a tight barrel etc. Sounds like you are only estimating velocity? Might be better to put it over a chrono.
Good luck with it. I'm a 7mm fan now, but was "married" to the 6.5-284 for quite a while, and its a great all-rounder. You'll be encouraged to learn that my best barrel was a Broughton 5C 1 in 8

. In fact I shouldn't say "was". I dusted the cobwebs off it for a local pennant shoot recently and it produced a couple of 60s!
Alan