The food your eating can start raising your BG before the insulin has a chance to work. It takes for most insulins about 20 minutes to start having an effect. I would suggest you discuss this with a professional because it can really vary from person to person.
I always prebolus (for coffee up to an hr. in advance and get a straight line on my CGM :-)) I do know an approximate amount of carbs how much I will be eating most of the time, if I eat more (much more likely) I adjust asap. Are you bolusing AFTER eating? As far as I know they only suggest that for pediatric patients cause their eating can really be unpredictable.