Try real black powder.
1st off - pellets & sidelocks don't go well together as a general rule. The sidelock design is made for loose powder so it can get back under the nipple for good ignition. With pellets you're asking the fire from the nipple to go through the nipple channel, turn 90°, travel throughanother channel, and still have enough heat to light a hard to light pellet.
The T-7 loose can get back closer to the nipple if you pour it down the bore then "bump" the breech area with the heel of your hand to help it, but it still takes more fire to light than regular old BP.
My T/C NewEnglander sidelock fires perfectly with BP every time. With "substitute" powders it usually does, but an occasional "hang-fire" does happen. If I plan to shoot "subs" I switch to a 209 conversion nipple and get much more reliable ignition.
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