It's the standard problem of devolving responsibility to individual action: the effect is so diffuse as to be negligible in all but the most extreme cases. In most cases, one wouldn't even be able to correlate change in sales with the boycott action.

On the other hand, actions like picket lines produce a clear relationship between the issue (look at the sign) and the action. Contacting advertisers and shareholders to get them to pull out their funding also has a direct effect, and connects more cleanly the effect to the cause.