I had the same issue with Stamkos in my game. After two seasons he became UFA (won the Art Ross previous season), but no team offered him a contract, because they didn't have the cap space. Remained unsinged until August. I signed him then. He was also not really willing to lower his salary expectations, despite not getting any offers for almost 2 months.
I noticed a funny little inconsistency. I really like that the all-star game has teams named after star players, like in real life. But there is a problem. For example, in my game there was a team Kessel (I think), but when the game was actually played, Kessel was not the captain of the team. Same thing with the other team, don't remember who it was.