It could be coil or condenser or points , what colour spark are you getting?? , if it’s yellowish then a component test is in order, are or were your points badly pitted/burnt
Personally I would bin your so called spark tester , they are the most misleading toy you’re likely to waste money on , especially on Villiers ignitions , which it has to be said are notoriously weak.
Yet again , just try connecting your lead directly to the plug and see what happens , remember all early Villiers HT leads had a simple brass direct connector….for a very good reason.
Andy.