That is odd (he says helpfully!) because I've noticed the same . I don't spray the lawn . It may be that they are susceptible to constant mowing and are living in conditions on any lawn that are iffy for them at the best of times and so it doesn't take much for them to die out. The meadow next door is left to its own devices at one end ; the rest is sprayed and grazed; that end is always a mass of buttercups (and a good many cowslips too), such that passers-by stop to take photographs.Evidently, it's not local conditions (or global warming!) that account for their disappearance here.