DPO75002SX won't trigger on pulses above 28 GHz

Hello! We have 4 DPO7500 scopes that can measure up to 50GHz. We are having a problem where the scope will not trigger correctly past 28GHz. We are sending a simple signal: 1 us pulse width and period of 2 us. When we bring down the frequency inside the pulse the scope does fine, but past 28GHz it begins to fail. Why is this happening?

The SX scope trigger system is specified to 25GHz. Above 28GHz, you can try timeout trigger but it's not guaranteed.