No longer assign event to certain calendar when rt-clk

Hi everyone. I have been using a shared calendar called WORK with some friends who also have Iphones. It has been working great. Suddenly, I cannot assign an event to WORK anymore when on my Macbook Pro using Calendar. It always appeared before. Not sure I have tried to perform this action since I upgraded to Mountain Lion until now.

Btw, assigning from the Iphone shows no problems.

Below you see WORK showing fine on the Icloud website

Below you see WORK appearing fine as usual in the list on my Macbook's Calendar application but it is not available in the right-click menu anymore. It always always was before.