For me and my balls, there are four things that hinder a good session:
1. Not being throughly hydrated. Pumping draws fluids into the balls. If there's not a lot of fluids, not much happens. Drink LOTS of water before, during and after a session.
2. Not being warm enough. With me and my long legs, it's critical that my legs and feet are warm. Every guy probably has a part of their body that gets cold first. Keep that part warm and wear warm clothes, unless it's during hot summer weather. If your heating bill isn't too high, turn up the heat. When I'm cold or my feet are cold, my boys shrink. If I pump while I'm cold, my balls seem to fight to stay tight to my body, instead of relaxing, hanging loose and getting bigger. Note that if you get too hot and start to sweat, drink even more water.
3. NSAIDS (Non-steroidal anti-inflammatory drugs). These are common, over the counter drugs used to treat pain, muscle aches, swelling and inflammation. Probably the most common ones are aspirin, ibuprofen (Advil) and naproxen (Aleve). These drugs, and drugs like them, reduce swelling. If I pump while taking these drugs, the swelling that pumping is supposed to produce just doesn't happen. I may swell a little, but not much.
4. Being tense. My balls don't seem to want to relax if I'm tense. I try to take a minute or so to just relax before pumping, and I try to stay relaxed while pumping. I don't watch the TV news when I'm tubed up and I don't try to get work done. I've also found that being impatient with my progress doesn't help, either. I also seem to get bigger when I pump with a buddy. I think it's just the relaxing conversation. A relaxing hot tub with a friend is ideal, as long as you stay hydrated.
Also, think of a pumping session like a practice session that athletes or musicians have. Sometimes, for unknown reasons, a practice doesn't go well. A good athlete or a good musician doesn't get discouraged, they just keep going to practice. Eventually, they figure out what works and what doesn't work. The key thing is to just keep at it and to keep listening to your body.
Your mileage may (and probably will) vary.