But too be fair, Sam only had it for a short time. Also in the movies he also struggles to give it back to frodo (I can't remember the books). Watch it again and you can see him hesitating and only giving it back when frodo prompts him harsher. Though you could interpret it as him not being sure whether it is a good idea for frodo's health that frodo gets the ring back. Same with bilbo where Gandalf had to influence him quite a bit.