In a few rifles, the flash channel that the fire must negotiate to get to the powder make it difficult. Take for instance a common CVA percussion. The fire is directed down through a constricted nipple, then must make a 90 degree angle and travel another half inch to where the powder is located. In a Thompson center, the fire goes down through a nipple into the breech plug and then makes an angle and must travel another 5/32nds long tunnel to get to the main charge power.
In an underhammer, the fire goes through the nipple and directly into the powder charge.
In an inline with percussion, the fire goes through the constricted channel in the nipple, but like the underhammer. goes then directly into the main charge powder.
So sometimes with more traditional configured percussion breech configurations no 11 caps do seem to have more difficulty igniting the fake powders. Often it is just a matter of keeping the flash channel open and clean.