It’s not uncommon for artists to have problems trying to remove caps from paint tubes they’ve previously used. This is usually caused by paint drying between the cap and the thread of the tube’s opening. If there’s acrylic paint on the thread of the tube after it’s been used, the paint acts as a glue between the tube opening and the cap once the cap is replaced.
To avoid this, simply give the opening of the tube a wipe with a wet rag and ensure that all the excess paint is removed from the thread before replacing the cap. This way the tubes will be easy to reopen next time.