If you are an Alpha Clone then the ability to accept Level 4 and Level 5 missions was removed from Alpha due to abuse as part of the March update.
From the March 12th release notes:
Missions & NPCs:
Level 4 and 5 Agent missions are now only accessible for Omega Accounts
If not, then log out, log back in. If it’s still an issue then raise a ticket from within the game - doing it from in game gives CCP some more information to work with. They won’t address issues in forum, the ticket system is the right place for this.
IMO it’s because the game caches the “no” from the agent for an infinite duration. That is, when the client asks the mission to the agent, if the agent says “non” the client then will always store that previous “no” and return that “no” instead of asking to the server.