My husband goes flat when he inserts himself in me,why?

How old are you both?

There's a lot of possible reasons, some physical and some emotional. Talking about it with him should give some clue (can he have an orgasm when alone, etc). If emotional, he needs to be gently listened to. If he can't or won't tell you, he could see a therapist. A few reasons: after the wife has a baby, or they are losing testosterone (very common), or are realizing they are gay, or have fallen for someone else, or think you had an affair, or after you were raped.

It helps to know ages because there is a HUGE difference between 20 and 60 as far as advice.

He is 53 I am 47

When was the last time he had a complete physical exam and that includes reviewing the medication he takes.

When was the last time he had a complete physical exam and that includes reviewing the medication he takes.

I agree... he is clearly at the age he might have a medical issue or issues that have slowly been developing that can cause this problem. He needs a complete and torogh physical and to discuss this with his physician so they can actively look for the cause.