Ok, about a month after my partner and I started having sex we had this problem. We would start to have sex and really be enjoying ourselves, but I could feel him strt to go soft and we would have to either stop completely or go really slow until he got hard again. This kept happening every time we had sex and it because an issue. Luckily, we are both very very open with each other and we talked about it and didn't let it become an issue. He did some research and found out it is called 'yo-yo-ing' and is common in men who are very stressed. My partner was going through a tough time in relation to his job and money and other various stresses, which is what causes it. It is a mental thing, not a physical thing. Make sure you talk to him, don't ignore it because doing that only makes a problem worse. Get him to open up to you and try and work through whatever is stressing him out; this should help.