I have one under my turntable. I use a bicycle pump.
Turn it upside down and then see if the feet move when you start to pump. It may just be sticky. You may not be getting a good seal with the pump you are using. You may also have a leak and it will be easier to track it down when you can see the inside of it.
After you determine where the issue is in the Vibraplane you can contact Sounds of Silence in New Hampshire and the can likely help you. They sell Vibraplanes primarily for audio use.
Turn it upside down and then see if the feet move when you start to pump. It may just be sticky. You may not be getting a good seal with the pump you are using. You may also have a leak and it will be easier to track it down when you can see the inside of it.
After you determine where the issue is in the Vibraplane you can contact Sounds of Silence in New Hampshire and the can likely help you. They sell Vibraplanes primarily for audio use.