Pglasgow

That seems to suggest that your powder isn't getting under the nipple. So I'd follow David's suggestion for cleaning out the breech. Roundball makes a good point about blackpowder, but if the sub is under the nipple it will ignite as you have found out. If you wish to use a sub, I have had the best success with Pyro P, which is of very fine granulation (which aids in filling under the nipple) and propells PRB, grain for grain, much faster than BP. For consistency and accuracy, blackpowder still wins for me, though.

Be sure to tap the butt of your rifle gently to help settle the powder under the nipple. When I do that step, I will tilt the rifle so the nipple is "downhill" of the breech to aid the flow there. Larger grains can more easily bridge, sothe finer granulation of subs will generally do better.