Could well be the foot on the plunger Peter, or a bit of wear on the notch on the CAMring. (Not the slipring, that the plastic bit inside that the brushes track on).
If the plunger is part-disengaging or jamming at one end of its travel or t'other, that's the most common problem.
Check the edge on the camring cable notch is good, and clean up if need be - very gently. If the plunger can't seat fully 'in', then it will jam in certain cases when operated - in which case tweak the mushroom foot to get a flush fit.
Most common problem is that at the limit of ring movement the plunger slips out of engagement because the camring geometry is off kilter. Could be the eccentric stop pin that controls camring movement could be adjusted a fraction to alter the ring's relative position to the housing - and improve the geometry of the plunger - but the pin's main purpose is to fine-tune the internal timing of the mag, not get round a cable / plunger problem. (Points should open just a tiny handful of degrees after the magnetic flip, at full advance - delay retards and weakens the spark, too early kills the spark as the coil won't have charged up). There is some leeway though and if a compromise is needed to get the camring to operate correctly, then so be it in such circs.
Worst case, if the problem persists, you could grind new notches on the opposite sides and 'start again' with the geometry. First thing to do there is establish where the ring wants to be at full advance (points just opening after flip, etc). It will be close to 180° opposite, but not quite (or we wouldn't be doing this!)
Cut a small notch big enough to fit the stop pin only. Offer up to housing, with eccentric pin in its central position. See how close to optimal the points opening position is ref the flip. If happy, enlarge notch to provide retard. If not happy, provide a tad more advance, check again and then grind for retard.
When happy here, cut new cable slot. Set camring to mid-position in housing and mark to cut slot so as to have plunger foot at mid-position too. Then you get best angles at full extent in both directions, and least chance of disengagement.
Fine tuning by the eccentric pin is still an option, but probably not necessary.
Best to do this, if you have to go this route, using a new opening cb point - as wear on the fibre heel affects the optimal setting of ring by a good few degrees. You wouldn't want to use a worn heel, set it all spot on, then find next time you changed the cb points for new the mag was over-advanced and sparkless. It happens!
Worth also ensuring that if there is meant to be a gasket stopping the camring from sliding axially out of its housing, it is there. Or that the little plastic footings on the inside edge of the plastic cover are making contact with the ring to keep it snugly in place.
It's quite a common problem to be honest, but can be exasperating - and a consequence of several small things rather than one obvious one.
