Im with Jody,

I think the CDI is defective,

It has to do with internal combustion pressure that makes the spark week it will show spark on the out side and it is hard to belive that it does not work when the plug is back in the head that was my experiance I did change the CDI and off I whent,
I had no problem with the stock plug on my DL 50 but I did change it after 3 months of use just to be sure it woud not fail on me at a bad time.