This is not unusual. It is one of the reasons some sidelocks have more ignition problems compared to inlines, particularly in very cold weather. Every time that flame turns 90degrees, it is cooled by the cold steel. One thing you might think about. Since you know your breech plug has a narrow channel, you can't clean it with a jag. You probably should set the base of your barrel in a bucket of very hot water and use your cleaning jag as a piston to pump hot water through this narrow channel. Any lube or oil you use can settle in this channel also, so you will need to be sure to shoot of a few caps to dry it befor loading. Loose powder shood fall into this channel some to help get the main charge going.