It happens to all brass players eventually, a stuck mouthpiece. No matter how hard you seem pull it stays lodged firmly in your instrument. The hardest part is getting it out WITHOUT damaging the rest of your instrument because tubas, trombones, trumpets, and whatever else you may play cost a lot of money and are quite delicate. IF YOU THINK YOU WILL ACCIDENTALLY DAMAGE YOUR INSTRUMENT, TAKE IT TO A REPAIR SHOP, and let someone who knows what their doing take care of it. Apparently there is an actual tool for pulling mouthpieces, but my band director didn't have one the right size for a tuba mouthpiece.
The scientific theory behind this strategy is simple: heat the lead pipe causing the metal to expand, while cooling the mouthpiece causing the metal to contract and come loose.