Normally the recommendation is that rapid acting insulin takes about 15 minutes to start having an effect. Those of us with libres have seen it take longer than this dependent on injection site.
The difficulty comes with what you are eating and in what combinations. Carbs, protein and fat together, similar to pizza or curry with rice often result in a slower increase that standard rapid acting insulins struggle with. You may need to split the bolus insulin.
With your example snack, what did you eat and how did you deal with it? Similarly, when you've been seeing these issues, what were you eating and what were your insulin doses?