Slick have an SB that recommends a 500 hour check, the reason for this is to change the contact breaker points and check the internal timing in the mag.
The internal timing has a large effect on the strength of the spark, while on most occasions the Slick mag will run OK up to about 700 hours the efficiency however drops of to the point that the engine won't run at idle on one mag.
It follows that a weak spark will not start an engine very will especially when cold it may well be the mag internal timing.