You are likely having issues due to the relatively low listed pressure and the chosen components. I had similar experiences with pressures under 9,000 psi in the past. This is especially true if the shells are left in an unheated area such as the vehicle or even gun for a couple hours or more. I've had this issue with target loads on up to hunting loads in 12, 20, and 16 ga guns.
To solve, it took changing to a load that produced pressures in the high 9000 range and preferably over 10,000 psi. I ended up using powders that were relatively fast burning for a given shot charge. Unique is a common powder I use for 12 and 16 ga 1 1/4 and 1 1/8 oz respectively. The 20 ga often gets Unique but Herco, WSF, Universal, and HS-6 are often used. Coupled with a Federal 209A or CCI 209M primer and using original factory wads rather than generic or clone wads I get good ignition to well below zero degrees.
DO NOT use Remington 209 or standard CCI 209 primers as they are among the "coolest" flame temp primers around. They are both notorious for poor cold weather performance. The same can be said for most of the European primers other than the Fiocchi 616 which is on the hotter side of the average but still not to the level of the Federal and CCI magnum.
The actual factory wads seem to not become brittle or otherwise have sealing issues which some of the substitutes do. Some of the clones are better than others in this regard but they all seem to be more sensitive than the factory OEM wads. I overwhelmingly use clones but for cold weather use I keep some original manufacture wads on hand.
As for Longshot use, I haven't loaded it in a shotshell in close to 20 years. I get bloopers and off sounding reports when I use Longshot in my 12-28 ga Chambermates. The same occurred with Remington STS factory loads of the era and was told that was due to Remington using Longshot for their factory loads. As I acquired a sizable amount of Winchester 540 and Hodgdon HS-6, that is what I am using for 28 ga. The Longshot is now used for mid-range 357 mag pistol loads. That is a good fit for now.
There was also mention of plated shot. As stated, copper "plating" is not really and the underlying pellet can be rather soft. Same goes for the nickel plated shot which often contains chilled shot. I stick with plain old magnum lead shot as it is a little more predictable in what one gets. It may pull in some feathers but I tend to use mostly 4 and 5 shot so I still get good penetration.
