Sometimes when I'm going solo I have a much harder time. And when I am able to, I'm usually only good for one orgasm. I don't really have much problem with it though, if I'm not in the mood I'm not and that's ok.

I've also noticed it changes depending on what part of my cycle I'm in.

Given the fact that you can achieve orgasm under certain circumstances, it seems like this is less a physical problem and more likely a mental block.

Sometimes when you are having sex with another person, focusing on the task at hand is easier because you are interacting with another person. Masturbation can sometimes be challenging if you have other things on your mind and simply do not focus on what you are doing. It's easy to suddenly find yourself planning the weekly menu or worrying about paying the bills if the stress of life is particularly high.

I agree with Airen that setting the mood and perhaps watching a video or reading an erotic book might help your brain get on the same page as your body.

I discovered that when I use the television as cover-noise, instead of music, I get distracted and spoil my good time. That's probably not an issue if you're with a partner.

I have noticed that unless I get myself really worked up first with either erotica or porn, I just don't even bother! Definitely have to have something to kick start it. Partners are always a plus but if not then find something that will really jump-start it then try solo?