Occasionally I'll get a tube or filter, etc that decides to get stuck. Most times with a significant amount of swearing, I eventually get the items apart.
However I've now found a solution that works well and doesn't mark the objects targeted.
It's an item called a "boa constrictor" that you can get from Bunnings. The pack I bought has two of them in it for different sized objects. You can get them separate if desired. You can see pics of them off their website:
http://boa-uk.com/
Just thought I'd pass this tip along as I found these stuck items most aggravating to deal with...