why cant he cum?

he has never had this problem. we are ttc for our rainbow baby. its being 4 months since we started ttc i ovulated yesterday . the past 2 days (yesterday and day before) we where having sex with good feelings but how ever he tries which ever position he could not come. i dont know y this happend any of u have any experience with this. on the 1st we had sex and he came in me ( we saw each other after a weak). please help