If you are not getting hard enough for penetration, your pump technique is possibly not quite right - it should work without any call on your blood flow properties.
Are you
- trimming the pubic hair in that area as close to your body as possible
- putting lube on your pubic area as well as inside the tube
- best technique is to pump slowly and stop every few pumps to give the blood time to arrive - it can take upwards of 20 minutes until you get more practised
- does the penis engorge fully in the tube and then droop when you remove the tube? If so, you are not using a tight enough ring - some find that a combination of 2 different size rings works best
- alternatively, are you accidentally releasing the vacuum a little when you transfer the ring from the tube to the penis?
What make and model of vacuum pump are you using?