I think the best lesson to learn for a guy new to the sport is that you are going to be caught. It's the frequency and severity of these punches which matter. This is why sparring is so important, you've identified the weakness in your defence and now you and your trainer go about rectifying it. I have had this problem too, the cause... I was unconsciously concentrating on his right glove rather than between his gloves. Made my reactions to his right great, but his left hook just didn't register as quickly. Good luck with it brother