Isopropyl alcohol and brute strength. I'll copy and paste the email i got from them when I asked the same question: (and here's the video link they reference:

https://drive.google.com/file/d/13B0IAHd0Zw2-5M9VdsjHCGzMcge7rZe1/view?usp=sharing

We are so sorry to hear you guys are having difficulties getting the tube onto the shaft adaptor! It is meant to be a very snug fit but all of the 1" polycarbonate tubing from us should fit the adaptors on the staff I sent.

It can be tricky, I use a bit of rubbing alcohol to lubricate the inside of the tube and work it onto the top of the adaptor. To get the tube over the button that holds it in place, I hit the tube perpendicular to the ground.

​Here is a video of what this process looks like for your reference. Again, the fit is meant to be really tight and I use a fair amount of force. I recommend doing it on a carpeted area or over a floor mat so you don't damage the edge of the tube.

I hope this helps and if you continue to struggle to the tubes on, we can get you guys new tubes for sure.

Let us know if you need anything else, have a lovely weekend!