To add the above, IMHO the main problem usually with some muzzleloaders and Blackhorn 209 (BH209) powder is the breechplug design.
A breechplug that works well with BH209 will have a flame channel with a diameter of 5/32" which is bigger than 1/8" which some plugs come with. This larger channel allows for more build up of carbon before you need to clean it to continue shooting. it also should reduce blowback. The tiny flash hole should be between .028 and .035 in diameter. When the flash hole gets larger than .035 it has been eroded and usually accuracy falls off and blowback starts to be a problem. A breechplug that has a removeable vent liner that can be changed when the flash hole erodes allows you to change an inexpensive component instead of buying a new breechplug.
If you have a good breechplug and use one of these primers - Win 209, CCI 209, CCI 209M, Federal 209, and still have ignition problems it could be that you are using a projectile or sabot that is to loose for good ignition.