For a time in 2004, LH instituted a policy of not allowing UA MP members to book award tickets earlier then 90 days of the current date (so you could not book an award ticket with a departure date after ~June 22nd today). This policy was later rescinded.

So LH may be launching a new policy on restricting UA MP awards on LH metal.

Or there may not just be any availability and the agent is giving you some cock and bull story to cover that lack of availability.

Or the agent may have heard a rumor to that effect, sees no award availability, puts two and two together, and comes up with five. In other words, this rumor may be unfounded, but they're assuming that it is not.