NO BABY MAKING GOING ON WITH THIS PROBLEM

Okay so just needing some advice. The past week my husband and I have been doing the deed just about every day are every other day (trying to conceive) well there has been three separate occasions were I am giving him head and he is hard well as soon as I stop so that I can get on top he goes soft (frustrating af) anyway can someone please give me some advice on what I am doing wrong