The key to good e-stim sessions is patience as well as experimentation. Everyone reacts slightly differently so for something that works extremely well for me may not be as effective in someone else.
Electrode placement is key to getting the most out of your e-stim system and as has already been stated if you feel hot spots or sensations at only one pad then there is a problem with conductivity there. You may have a set of cheap pads (it does happen). Here are a few tips:
1. Make sure that you are thoroughly hydrated, it might seem obvious but it can make a huge difference to how things feel.
2. Ensure good adhesion of a pad style electrode, I place mine on skin without any hair (yes I shave down there).
3. Keep an e-stim diary of settings that you have used and where you placed your electrodes. This can make it easier to avoid something you have tried before and also makes it easy to recreate amazing sessions.
4. If something didn't work for you try it again on another day as stress, hydration and other things can affect play.
5. Don't fixate on the hands-free orgasm, e-stim can be an amazing way to give and receive pleasure even if it doesn't result in an orgasm.
6. Remember that you can always increase stimulation with other toys. I can recommend a wand style vibrator if you are having problems getting over the edge.
7. There are lots of good resources concerning electrode positioning guides, you will have to google them though as I don't want to appear to be spamming the forums.
Take your time, experiment and just remember to have fun.