If the internal timing is advanced too much (by having a large gap) the points may open before the field has collapsed – resulting in no spark at all. The physics of a magneto's method of operation means that there is some automatic advance as the revs go up. It may be that a magneto with too much advance will spark at cranking/kickover speeds and low revs but this additional automatic advance as the revs increase may be enough to lose the spark altogether at higher revs.
If the internal timing is retarded too much (either by having a reduced gap and/or by operating the advance/retard mechanism) the rate of magnetic field collapse will have slowed down by the time the points start to open so the spark will be less intense. Magneto efficiency increases with speed of rotation so this will be less apparent at higher revs but on a 'tired' magneto starting may be difficult and it is not unusual to have no spark at all at full retard at cranking/kickover speeds. (Watford, Splitdorf and Scintilla, amongst others, found a way to get round this but that's another story).