A couple of people have sent me a PM since I posted above and I don't like replying one to one in case the wrong information is given and there is no-one else to see it and correct it (personally, I think the biggest mistake of the new forum was to introduce PMs)
Common mistakes are to not mix it well enough or to forget to knock out bubbles from the top of the needle before injecting, or to inject into the wrong part of the penis. To inject Caverject correctly, the needle should go in half way down the shaft at a good 90 degrees - if you angle it even slightly, you might miss the spongy tissue and it won't work properly. Avoid visible veins or arteries as these will carry away the solution. Give the penis a massage to help the solution move around the spongy tissue. It is best to alternate from right side to left side so that scar tissue doesn't build up. You would know if you accidentally injected into the urethra as the solution comes out of the end of your penis.
Sometimes, there are rogue injections in the box or even a whole batch can be faulty. John once had a box in which every single one failed but it is possible they had been left on the windowsill and overheated.
If caverject is going to work, it will work regardless of whether you are in the mood or not. If it seems to work and then deflates, don't make the mistake of thinking the opportunity has been missed. The first time John had it with the ED nurse, he got an erection immediately but we then had a row and it went down. We drove home, stopped at a cafe, had another row and eventually kissed and made up, at which point the erection returned - this was probably about 90 minutes or more after the injection had been given. A bit of self-belief seems to help it along :-)
Sadly, there are quite a lot of men for whom it doesn't work at all but if it worked when the ED nurse did it then it is most likely a technique issue