If your using a very wet patch to do your barrel wiping, that can cause a problem in the vent liner. Also, your method of wiping the bore can cause a problem.
First off, the wet patch should only be slighter wetter than "damp". If it has too much liquid on it, the liquid will be deposited in the cone on the inside of the vent plug. There, the liquid will wait until some nice dry powder from the new load comes in contact with it. The powder will become soaked with the liquid and then act as a hard to ignite block to the flash from the pan.
If you are pumping the wiping patch up and down the bore, that is almost guaranteed to shove wet fouling into the cone inside the nipple. This will be an even more effective dam against the flame from the pan flash because where the slightly wet powder can ignite, damp fouling can not.
The right way to wipe the bore in my opinion, is to run the damp patch completely down to the bottom of the bore and let it rest there for at least 5 seconds. Then, in one smooth action, pull the damp patch up completely out of the bore.
The 5+ second wait time will allow the moisture from the damp patch to soak into the fouling in the bore and make it soft. Then, when the patch is pulled out of the bore it will wipe off almost all of the wetted fouling.
Follow this with a dry patch to dry the wetness that was left in the bore and then reload.